Auto electrical advice required

Submitted: Monday, May 15, 2006 at 21:31
ThreadID: 33927 Views:2836 Replies:7 FollowUps:8
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Advice is required on the probability of damaging one of my batteries - I have an imported Nissan Safari with a 24 volt system. Have had a stepdown transformer fitted to run my caravan and electric brakes with the UHF also linked into the 12 volt supply. Since then I have fitted an MP3 player and found that the OEM step down transformer could not supply enough power (10A fuse) so have run a new feed direct from the first battery giving me 12V at the 15A required.
What will the consequences of this set up be - will it cause issues of low charge on this first battery or indeeed the second one in thats in series?

Your thoughts would be appreciated
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Reply By: gen3rules - Monday, May 15, 2006 at 23:21

Monday, May 15, 2006 at 23:21
Doing this will cause your batteries to become unequal in charge and the drawn battery will stay flat while the other will recieve too much charge. you will reduce the life of you batteries considerably as well as having starting problems. Another reducer is the best option and only costs the price of the battery you will damage by drawing off it.
AnswerID: 172877

Follow Up By: Marn - Tuesday, May 16, 2006 at 06:46

Tuesday, May 16, 2006 at 06:46
The battery that is drawn off will have a lower voltage than the other thus when the charging voltage is applied the flatter one will have a greater differential and therefore take more current until both battery voltages are equal. No battery will be overcharged. There have been many systems around like this for years and i think you will find that the early 24v nissans were set up like this
Gerard
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FollowupID: 428580

Follow Up By: Member - andrew B (WA) - Tuesday, May 16, 2006 at 07:39

Tuesday, May 16, 2006 at 07:39
yep, my MQ had the 12 volt drawn off the first battery. You could possibly draw 12 volts off the 2nd battery for some of your gear to even out the draw (use a return earth to the same battery. Alternatively, if you have the room, a second 12 volt alternator and battery can be a great aux battery totally independant of the 24v starter system.

Chers Andrew
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FollowupID: 428586

Reply By: traveller2 - Tuesday, May 16, 2006 at 08:11

Tuesday, May 16, 2006 at 08:11
The easiest and most reliable way is to fit a Redarc Charge Equaliser, you can then pull any amount of 12v from either battery and it keeps them both at the same potential. No loss or wasted power through stepdown transformers either. also no restriction on power drain on 12v.
Think of two water tanks with a pipe between the bottom of both, any water removed from one will be topped up by the other through the equalising pipe.
I've had one fitted for over 4 years on my 24v vehicle with zero probs, actually just replaced the batteries as one dropped a cell, they were 4 years old.
No connection to Redarc just a satisfied customer.
AnswerID: 172896

Reply By: Member - Charlie M (SA) - Tuesday, May 16, 2006 at 18:09

Tuesday, May 16, 2006 at 18:09
Need to have charge equalizer fitted or will always have trouble and be replacing batteries every 12months to 2 years.
Cheers
Charlie
AnswerID: 172979

Follow Up By: Mike Harding - Tuesday, May 16, 2006 at 18:35

Tuesday, May 16, 2006 at 18:35
I tend to agree.

I sketched out the circuit and it looks to me as if the battery from which you are _not_ drawing the 12V will have to absorb more charge than it wants to which would lead to over charging (heat build up) of that battery and early failure as Charlie describes.

I'll stand to be corrected on this as it's not a trivial issue to analyse the current flow here.

Mike Harding

mike_harding@fastmail.fm
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FollowupID: 428677

Follow Up By: Marn - Tuesday, May 16, 2006 at 20:14

Tuesday, May 16, 2006 at 20:14
G'day mike. I see what you are thinking about the second battery, but, as the battery that has not been drawn from would have a higher voltage in it, it just would not accept the charge would happily sit there and recieve a trickle charge
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FollowupID: 428722

Follow Up By: Mike Harding - Tuesday, May 16, 2006 at 20:32

Tuesday, May 16, 2006 at 20:32
I'm not sure Gerard.

If we assume the battery providing the 12V (battery 1) is the one with it's negative connected to 0V then current from the alternator has to flow through the other battery (battery 2) in order to charge battery 1. Now, if battery 1 is 50% discharged and battery 2 is 100% charged will battery 2 dissipate any power from the current forced to flow through it by the alternator and will the alternator regulator have to raise it's voltage in order to force sufficient current into battery 1 thereby causing battery 2 to accept more charge than it wants to?

My brain hurts Brian :)

Mike Harding
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FollowupID: 428734

Follow Up By: Mike Harding - Tuesday, May 16, 2006 at 20:36

Tuesday, May 16, 2006 at 20:36
Or will battery 2 go into some kind of load sharing and start to dump it's charge into battery 1? I think I'm going to have to Spice this....

Mike Harding
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FollowupID: 428739

Follow Up By: Marn - Tuesday, May 16, 2006 at 22:41

Tuesday, May 16, 2006 at 22:41
I tend to lean towards the load sharing idea. S ort of in one end, out the other. not held if you know what i mean. HMMMM.......Brain pain
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FollowupID: 428825

Reply By: bob&loz - Tuesday, May 16, 2006 at 20:27

Tuesday, May 16, 2006 at 20:27
I used this system for many years without problems except that if your first battery does fail or go flat you could get reverse polarity through the equipment you have connected to the first battery. use a one way diode in the + wire

bob
AnswerID: 173008

Follow Up By: Marn - Tuesday, May 16, 2006 at 22:42

Tuesday, May 16, 2006 at 22:42
Good point.
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FollowupID: 428826

Reply By: Member - John C (WA) - Tuesday, May 16, 2006 at 21:25

Tuesday, May 16, 2006 at 21:25
Thanks guys for your input. Obviously when using the caravan via the stepdown transformer and or the engel hooked to battery 2 it will help the situation. For now I will swap the batteries over every few months.
AnswerID: 173022

Reply By: gen3rules - Tuesday, May 16, 2006 at 22:51

Tuesday, May 16, 2006 at 22:51
whenever you have two batteries in series they must be of the same potential to be charged correctly and prolong the life of the batteries. Once you drain one it will not automatically come back up to the same level as the charged battery. The same amount of charge goes through both batteries so one will always be more charged than the other. If you only have a uhf or some such drawing you may get lucky and still get reasonable life, but only maybe. Also any auto electrician who puts a new battery in series with an old one is decieving the customer because its life will be cut drastically. The only way to do this is to fit another reducer or as above fit a charge equiliser both will work ok. Swapping the batteries around wont work unless you take them out on a regular basis and charge them both to the same level. If you dont fit a reducer or charge equiliser you will have more problems than it is worth.
AnswerID: 173042

Reply By: Member - Charlie M (SA) - Wednesday, May 17, 2006 at 09:44

Wednesday, May 17, 2006 at 09:44
When drawing 12volt off one battery that battery will be low charged on 24volt system. When engine is running alt will bring batteries up to required surface charge and regulator will turn charge off. Equals one battery with surface charge of 13.8volt and one battery fully charged of 13.8volts. When starting vehicle on 24volt one battery fully charged and one partly charged, resulting in battery overload, poor voltage to starter motor ect ect ect. Charge equalizer only way to go.
Cheers
Charlie Milton
AnswerID: 173100

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