Dual batteries for dummies

Some useful articles on doing it yourself
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Fri Jan 11, 2013 8:15 am

During my recent trip, I had some time to research the dual battery issues.

I am not an expert and any additions, corrections will be welcome.
The graphics is how I see the situation.

Points to remember:
A battery is like a earthen dam, with the top half containing more volume than the bottom.
The voltage regulator will decrease the current as the main battery`s voltage increases - to save energy [read fuel]
The voltage regulator monitors the voltage at one point - normally close to the main battery.
This means that the aux battery will receive a very low charge if the main battery is full, or close to full, even if the aux battery is empty. It therefore seems that the best way to connect these two batteries is immediately when the engine starts, and not to wait for the main battery to receive a charge first.
Main full.jpg
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The voltage regulator will increase the current when the voltage is low at the monitoring point, ie the main battery is low.
When other equipment is drawing current, aircon, fan, lights etc, the voltage regulator will increase the current until the maximum the alternator can supply. at this point, the voltage will drop and charging to the batteries will decrease or stop.
Main low.jpg
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With the engine off, the main battery is isolated by a relay [NL switch] and the aux battery is running the beer cooler.
engine off.jpg
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Current will only flow to the batteries if the voltage is high enough to reverse the chemical process of discharging. Any losses in the cabling to the aux battery wil decrease the voltage and reduce the charge to that battery.
The dc to dc charger increases the voltage to the aux battery and charging can again take place.
The dc to dc charger also uses technology to charge the aux battery properly such as the Benton or Ctek chargers does.
dc to dc.jpg
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The main [starter] battery gets a short, high current, discharge when the vehicle starts. The charging of this battery can be done in the same method.
The aux battery gets discharged slowly over a long period and needs to be charged similarly. [especially deep cycle batteries]

Solar panels is the best way to reduce the discharge, or recharge fully, the aux battery when the engine is off and the vehicle is stationary for long periods.
solar.jpg
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Two aux batteries connected in parallel will suffer the same consequences as the main and single aux, as the voltage monitor only sees the fullest battery`s voltage. The one that was drained more [due to mechanical differences] may not receive a charge at all and can drain to the point of permanent damage.
Using thick cables connecting these batteries have little advantage as current can only flow between these batteries when there is enough voltage. This means that equalisation between these two batteries will only take place until the fuller battery cannot provide the voltage anymore.

I hope this will clarify some of the mysteries.
I have said nothing about the charging methods of the different type of batteries as this is a totally different subject and the ac to dc and dc to dc chargers sort this out anyway. [if the correct one is used]

Hope the beer stays cold!!

Andre

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Fri Jan 11, 2013 8:50 am

Andre thanks for the explanation, but being a blonde girl, it is still as clear as mud to me... I see you and Marlize will be joining us at trekos, and if you don't mind I would appreciate it if you can explain it to me there. I have 2 deep cycle battery system and national luna thingie in my 4x4 trailer and it has been useless, I think that when it was installed by the 4x4 fundi's it was incorrectly wired. I have bought a 105 amp solar panel,to try insure the batteries remain charged but not sure whether the original wiring in the trailer needs to be changed. Think the batteries run parallel and possibly only the one battery is being charged and discharged. Won't be taking the trailer to trekos but will maybe try and draw the setup for you.
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Fri Jan 11, 2013 12:04 pm

Tonto wrote:During my recent trip, I had some time to research the dual battery issues.

I am not an expert and any additions, corrections will be welcome.
The graphics is how I see the situation.

Points to remember:
A battery is like a earthen dam, with the top half containing more volume than the bottom.
The voltage regulator will decrease the current as the main battery`s voltage increases - to save energy [read fuel]
The voltage regulator monitors the voltage at one point - normally close to the main battery.
This means that the aux battery will receive a very low charge if the main battery is full, or close to full, even if the aux battery is empty. It therefore seems that the best way to connect these two batteries is immediately when the engine starts, and not to wait for the main battery to receive a charge first.
Andre
Hi Andre,

Interesting! The voltage reg you're talking about, is that the NL one?

I must confess, this whole thing where the intelligent charger only "sees" the highest voltage of the 2 batteries and then doesn't charge the flat, aux battery I still don't understand. A simple charger (or alternator) doesn't discriminate, and will try and charge both batteries at the same time (at different rates according to the SOC of the batteries). Also, the voltage reg in an alternator will not limit the current (the plder ones anyway), and if you load the alternator too much (more than 2 flat batteries plus light plus plus), then the alternator will try to supply current untill it overheats and burns out.

I have a BEP marine switch in mine (with current monitor on both my main and aux batteries), and if the aux battery is flat, and that switch kicks in, there a lot of amps (up to 45 Amp for a few seconds), thereafter it will drop down to 25 Amp. If the alternator is not turning, and I override my switch, the 2 batteries will be connected and current will flow from the one that have the higher voltage to the one with the lower voltage (they will equalise), that's why I don't understand this whole "only the higher one gets charged" thing.

The switch will only kick in if any of the batteries voltages is above 12.7 v (alternator is charging).
Tonto wrote: Using thick cables connecting these batteries have little advantage as current can only flow between these batteries when there is enough voltage. This means that equalisation between these two batteries will only take place until the fuller battery cannot provide the voltage anymore.
I'm not sure I'm with you here. Thin cables are voltage thieves, then you'll need more voltage (difference) for the same charging current. Thinner cables will mean longer charge time.

Cheers,

Chris
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Fri Jan 11, 2013 1:55 pm

With these schematics I try to explain what happens when 2 batteries are connected to one charger.

Due to the complex chemical processes involved, of which I have little knowledge, I tried to simplify the explanation.
After reading many forums and some articles on batteryuniversity.com, here goes:

When 2 batteries are connected in parallel, the combined voltage will be slightly lower than the battery with the highest voltage.[this has been tested, or you can test it yourself][The 2 check valves connected to the gauge attempts to explain this phenomenon}
A lot of explanations and articles has been written on the modern intelligent chargers and how they work.
Essentially, the charger sees this voltage and due to the built in intelligence, makes a decision on what to do.
With a low voltage, the charger will go to the first stage, and charge accordingly. The charge will be accepted by the battery with the relevant voltage.
50 percent.jpg
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The battery with the lower state of charge gets no or very little [Difficult to show] of the charge. Basically the voltage needs to exceed a certain level before the reversal of the chemical process takes place.

As the voltage increases, the charger goes to the next stage of charging.
80 percent.jpg
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The empty battery still gets nothing.

When reaching an almost full level, the charger goes into a trickle phase, which basically means very little current is now flowing. The voltage during this stage is higher and can also flow to the low battery. The flow is however low.
100 percent.jpg
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The charger will eventually switch off, or go to standby when the voltage reaches a high enough level.
At this stage some equalisation will take place, up to the point where the voltage is too low for the current to flow.
charger off.jpg
charger off.jpg (90.06 KiB) Viewed 5464 times
At this stage I will appreciate if the real experts on this subject can join in and lets get this sorted once and for all.

The box at the end is Donkey`s cold beers :D:

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Fri Jan 11, 2013 1:57 pm

GI Jane wrote:Andre thanks for the explanation, but being a blonde girl, it is still as clear as mud to me... I see you and Marlize will be joining us at trekos, and if you don't mind I would appreciate it if you can explain it to me there. I have 2 deep cycle battery system and national luna thingie in my 4x4 trailer and it has been useless, I think that when it was installed by the 4x4 fundi's it was incorrectly wired. I have bought a 105 amp solar panel,to try insure the batteries remain charged but not sure whether the original wiring in the trailer needs to be changed. Think the batteries run parallel and possibly only the one battery is being charged and discharged. Won't be taking the trailer to trekos but will maybe try and draw the setup for you.
Will do Val,

Maybe my next post will explain it better.

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Fri Jan 11, 2013 2:06 pm

Andre... your explanation was very good, must be honest I have scoured this forum for info on dual battery system and read them all, and I have just come to the conclusion that I am seriously dumb when it comes to this dual battery, charger, voltage, solar panel etc. etc. stuff, I just get confused.. to many thingies everywhere.. Even have a second battery in my hilux and run a fridge off that when not using my trailer, and even then find the fridge runs low and not sure how to make sure the battery recharges.. all too confusing..
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Fri Jan 11, 2013 2:09 pm

Andre, as Chris says. The normal Voltage Regulator just regulates voltage, not current. Each battery will draw current as determined by its state of charge.

However, in a DB system, there is usually a timer circuit to allow the main battery to receive charge for, say, five minutes, and then the auxillary battery will receive its charge once the main battery is sufficiently charged. Therefore, you will require good cables (16mm minimum) to the second battery in order to alleviate voltage drop. Probably thicker cables if the second battery is installed in the canopy.

Nice way of explaining it with the water drop levels though!


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Fri Jan 11, 2013 2:13 pm

Hi Andre,

Interesting! The voltage reg you're talking about, is that the NL one?
The NL switch is the Solenoid or the marine switch connecting the aux battery to the vehicles charging system.
I must confess, this whole thing where the intelligent charger only "sees" the highest voltage of the 2 batteries and then doesn't charge the flat, aux battery I still don't understand. A simple charger (or alternator) doesn't discriminate, and will try and charge both batteries at the same time (at different rates according to the SOC of the batteries). Also, the voltage reg in an alternator will not limit the current (the plder ones anyway), and if you load the alternator too much (more than 2 flat batteries plus light plus plus), then the alternator will try to supply current untill it overheats and burns out.
The charger will only do what the voltage in the total system dictates, it does not know how many batteries are connected.
I have read tests where full and empty batteries are connected. the voltage reads only slightly less than the full battery.
I have a BEP marine switch in mine (with current monitor on both my main and aux batteries), and if the aux battery is flat, and that switch kicks in, there a lot of amps (up to 45 Amp for a few seconds), thereafter it will drop down to 25 Amp.
This is most likely the staring battery that gave a lot of current in a short period of time taking it back from the alternator. After a while, the voltage increases and the current drops. Exactly as I understand it.
If the alternator is not turning, and I override my switch, the 2 batteries will be connected and current will flow from the one that have the higher voltage to the one with the lower voltage (they will equalise), that's why I don't understand this whole "only the higher one gets charged" thing.
See below


The switch will only kick in if any of the batteries voltages is above 12.7 v (alternator is charging).
Tonto wrote: Using thick cables connecting these batteries have little advantage as current can only flow between these batteries when there is enough voltage. This means that equalisation between these two batteries will only take place until the fuller battery cannot provide the voltage anymore.
I'm not sure I'm with you here. Thin cables are voltage thieves, then you'll need more voltage (difference) for the same charging current. Thinner cables will mean longer charge time.
I agree with the thinner cables stealing the voltage. however, it seems no matter how thick you make the cables, in the end if there is not enough voltage, the current will not flow. This is apparently due to the chemical processes and one cannot see the batteries purely as a storage dam that will equalize fully.

Cheers,

Chris[/quote]

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Fri Jan 11, 2013 2:19 pm

Sorry just read your second post and diagrams, and that is pretty much what the guy that sold the solar panel to me explained to me.... I am going to take a photo of the connection in my trailer of the way the batteries are connected, ie when I can move it, and then maybe you can tell me whether it is connected correctly. Another issue which I have read conflicting opinions on is, when I am not using the trailer and it is standing in the garage, should I keep it connected to the electricity to keep the batteries charging or charged all the time, or not... and should I keep my NL 90l fridge/freezer running all the time or switch it off when not in use. The same question goes for the other battery and 40l engel I use in my hilux, which I take out of the bakkie when not in use?
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Fri Jan 11, 2013 2:24 pm

Family_Dog wrote:Andre, as Chris says. The normal Voltage Regulator just regulates voltage, not current. Each battery will draw current as determined by its state of charge.
I also think the voltage reg must have some form of current limiter, as the alternators does not seem to burn out that easily. I found that with too much current drawn, the voltage drops, at least on my system.
However, in a DB system, there is usually a timer circuit to allow the main battery to receive charge for, say, five minutes, and then the auxillary battery will receive its charge once the main battery is sufficiently charged. Therefore, you will require good cables (16mm minimum) to the second battery in order to alleviate voltage drop. Probably thicker cables if the second battery is installed in the canopy.
Personally I think it might be better to switch the second battery on immediately, as the combined voltage will be lower and more current will flow for longer periods. also to the second batt. In my double battery system the cables between the two aux batteries were very thick and very short.[25 mm thick, 300 mm long] One of the 2 batteries still died. This tells me that it did not receive a proper charge. This is what got me thinking in the first place. These 2 aux batteries were installed in the load box with 16 mm cable connecting them to the alt.
Nice way of explaining it with the water drop levels though!

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Fri Jan 11, 2013 2:30 pm

GI Jane wrote:Sorry just read your second post and diagrams, and that is pretty much what the guy that sold the solar panel to me explained to me.... I am going to take a photo of the connection in my trailer of the way the batteries are connected, ie when I can move it, and then maybe you can tell me whether it is connected correctly. Another issue which I have read conflicting opinions on is, when I am not using the trailer and it is standing in the garage, should I keep it connected to the electricity to keep the batteries charging or charged all the time, or not... and should I keep my NL 90l fridge/freezer running all the time or switch it off when not in use. The same question goes for the other battery and 40l engel I use in my hilux, which I take out of the bakkie when not in use?
Val, I think switch off the fridge. all you need to do is to keep all batteries charged when not in use. this is probably done best by disconnecting them and alternately charging them with an intelligent charger.

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Fri Jan 11, 2013 3:57 pm

Tonto wrote: The voltage regulator will decrease the current as the main battery`s voltage increases - to save energy [read fuel]
The voltage regulator monitors the voltage at one point - normally close to the main battery.
Andre
Hi Andre,

Which voltage regulator is this?

Do you have a NL system?

Cheers,

C
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Fri Jan 11, 2013 4:35 pm

Tonto wrote: I also think the voltage reg must have some form of current limiter, as the alternators does not seem to burn out that easily. I found that with too much current drawn, the voltage drops, at least on my system.
Hi Andre,

I don't know if the newer types of alternator have current limiting capability, but the older ones definitely didn't have any. Voltage drop is because of resistance of the conductors x the current drawn(IxR). It's also called IR drop. It's not a constant and the amount depends on the load (current) drawn. The alternator stator windings also has internal resistance, and if the load (current)is high, the IR drop will increase in the alternator windings as well as all the other conductors, so thats what's causing the voltage drop. Too high a load will cause the alternator windings to get too hot (heat = I^2 x R), insulation will break down and the alternator will be cooked. The alternator will just keep on trying to satisfy the load (current) demand without any controlled current limiting.

Cheers,

Chris
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Fri Jan 11, 2013 4:58 pm

cprinsloo wrote:
Tonto wrote: The voltage regulator will decrease the current as the main battery`s voltage increases - to save energy [read fuel]
The voltage regulator monitors the voltage at one point - normally close to the main battery.
Andre
Which voltage regulator is this?
The regulator on the alternator
Do you have a NL system?
A similar system with a Cole Hersee solenoid I can switch on manually, to connect the aux to the vehicles charging system

Cheers,

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Fri Jan 11, 2013 5:00 pm

cprinsloo wrote:
Tonto wrote: I also think the voltage reg must have some form of current limiter, as the alternators does not seem to burn out that easily. I found that with too much current drawn, the voltage drops, at least on my system.
Hi Andre,

I don't know if the newer types of alternator have current limiting capability, but the older ones definitely didn't have any. Voltage drop is because of resistance of the conductors x the current drawn(IxR). It's also called IR drop. It's not a constant and the amount depends on the load (current) drawn. The alternator stator windings also has internal resistance, and if the load (current)is high, the IR drop will increase in the alternator windings as well as all the other conductors, so thats what's causing the voltage drop. Too high a load will cause the alternator windings to get too hot (heat = I^2 x R), insulation will break down and the alternator will be cooked. The alternator will just keep on trying to satisfy the load (current) demand without any controlled current limiting.

Cheers,

Chris
Accepted, I will watch the system and switch off as needed.

Reg.

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Fri Jan 11, 2013 7:36 pm

Tonto wrote:
cprinsloo wrote:
Tonto wrote: The voltage regulator will decrease the current as the main battery`s voltage increases - to save energy [read fuel]
The voltage regulator monitors the voltage at one point - normally close to the main battery.
Andre
Which voltage regulator is this?
The regulator on the alternator
Do you have a NL system?
A similar system with a Cole Hersee solenoid I can switch on manually, to connect the aux to the vehicles charging system

Cheers,
Hi Andre,

The regulator usually sits inside the alternator, and it monitors and regulates the voltage inside the alternator. The regulator ignores all external IR drop (between the alternator and the battery), which is actually unfortunate, because if the voltage measuring point was to be on the battery itself, all our problems would be solved!!! With bigger load the internal IR drop of the alty will cause the measured voltage to drop, then the reg will increase the field current to get to the reference voltage again.

In any case, very interesting about the batteries in parallel that doesn't get charged! I have batteries, Benton etc, I'll do some tests. I also have a shunt to measure current, should be interesting!

Cheers,

C
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Fri Jan 11, 2013 9:28 pm

Hi Andre,

The regulator usually sits inside the alternator, and it monitors and regulates the voltage inside the alternator. The regulator ignores all external IR drop (between the alternator and the battery), which is actually unfortunate, because if the voltage measuring point was to be on the battery itself, all our problems would be solved!!! With bigger load the internal IR drop of the alty will cause the measured voltage to drop, then the reg will increase the field current to get to the reference voltage again.

In any case, very interesting about the batteries in parallel that doesn't get charged! I have batteries, Benton etc, I'll do some test. I also have a shunt to measure current, should be interesting!

Cheers,
I agree, that seems to be the main problem added to the fact that we mostly use two different type of batteries [high and deep cycle] which needs a different method of charging. The clever chargers available takes this into account. Apparently, if a lead acid battery is discharged, it needs to be recharged over a period of about 14 hours. Much longer than I would like to drive in a day! The best thing to do would be to prevent the battery from discharging as far as possible and recharge fully as soon as you get a chance.
When stationary, or only driving for a few hours daily, a solar panel will help, as well as a dc to dc charger to recharge the aux battery as well as possible. Using multiple aux batteries seems to work only until they start to run down, then the problems start.
It will be very nice if you can test these theories, as a lot of money is spend on these type of systems with unsatisfactory results.

Regards

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Fri Jan 11, 2013 10:25 pm

Nice comparison!,
Volts in electrical system is like the pressure (height of the reservoir) in a water pipe from a main reservoir (the alternator) , and the current (amps) the size of the pipe or stream from the pipe to a second reservoir on a slightly lower level. The regulator keeps the volts constant from the alternator as it runs at various speed from the engine, not to cook the battery at higher engine speed, normally about 13.8V to 14V. The size of the alternator determine the amount of amps delivered.
As the second reservoir fills the height differences get less thus the flow of water reduce, same principle on charging a battery.

Proper deep cycle batteries can't take the the higher amps from the alternator and needs to charge at a bit higher volts as a start battery (13.8V) , so no use in in a solenoid/and or DB system.

Semi deep-, AGM or lead crystal batteries seem to be working better in a DB system as can be charge with DC to DC charger sufficient in a reasonable driving time.
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Sat Jan 12, 2013 9:03 am

I am also looking at improving my system & received this mail from Warwick Lesley at Alucab in Cape Town when I approached him about fitting a second Solar Panel, to assist with the Charging of the second battery.

"The panel running in Tandem with the alternator will not be a major help as the amperage supplied is relatively nominal, in my humble opinion I would tackle this whole exercise differently as there are some very nice dual battery controllers that are now on the market that really change the way battery systems perform. In a nut shell the conventional solenoid systems do not effectively charge the 2nd battery, but the new Ctek 250s dual does, as a quick example, if you take a flat 105ah battery and charge via solenoid to reach 75% capacity (the ceiling with these systems) will take 8 hours, if you take the same battery and charge via a Ctek 250s dual it will take about 2.5 hours to reach 95% !! 20% extra effectively 4 odd hours of a 100% efficient 80w solar panel input!! The ctek also acts as a MPPT solar regulator, which in simple English is far superior to a conventional solar panel and you will get much more efficient service from you current panel, so this is where I would start and then add the second panel if need be or as well".
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Sat Jan 12, 2013 4:12 pm

Agreed, something we maybe dont understand always that a battery is a complex piece of kit and the chemistry of the aux batt normally differ from the starting batt. The discharge method dictates the battery type and the normal alternator is designed to handle a high cycle battery discharged with high current over a short period of time.
Add to that the modern silver ion batteries fitted in the hope to get a better battery on older vehicles. These batteries mostly need a higher voltage to charge them properly, as does the AGM deep cycles etc.
It all becomes very confusing, but the only method to sort the lot out seems to be a dc to dc charger. All the logic is built into these units and they seem very easy to fitt. No relays, solenoids etc.

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