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Chopperp Guru Joined: 03/01/2018 Location: AustraliaPosts: 1057
Posted: 07:48am 26 Jul 2018
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I know that there is a fairly common practice that if you want 12V from a 24 or 36 volt supply etc that you simply tap across the lower 12V battery. I've actually seen a commercially designed small 36V electric vehicle with 12V for the horn, lights etc taken from across 2 of the 6, 6V batteries they use.
In my opinion, this is not a good practice as the battery(ies) supplying the 12V gets discharged more quickly than the others, subsequently has a lower voltage across it which in turn means a higher voltage across the remaining ones. This ends up as a not so good situation if you want to keep the batteries for reasonable time.
I was just wondering what others think about the practice of tapping multiple batteries. Would this be a viable solution in some circumstances?
The reason I ask is that we had a situation today where to solve a problem with a 12V engine driving a 24V alternator & to minimise batteries, we could just tap off one 12V battery for engine starting. The normal 12V charging circuit for the engine would be disconnected & the 24V alternator would charge both. ("This is common in the marine industry" was the quip I got.
T.I.A. ChopperP
Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498
Posted: 08:49am 26 Jul 2018
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Not a good idea unless each battery is charged independently, but if it is only a very small draw it may not matter. There are lots of step down converters on Ebay, Aliexpress etc and there 10A and above ones so there is no need to tap into the middle of the battery bank. There are only 10 types of people in the world: those who understand binary, and those who don't.
noneyabussiness Guru Joined: 31/07/2017 Location: AustraliaPosts: 513
Posted: 09:31am 26 Jul 2018
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If its only a short term load, like a starter , or a low load, then there would be minimal difference.
I have personally seem this done many times, for the above issues. But as said, if there is a significant difference then it will have balance issues. However if flooded lead acid they are pretty hardy and will usually balance out pretty well and quickly. I think it works !!
Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814
Posted: 09:52am 26 Jul 2018
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It's not a good idea, but still done because it's "easy" and cheap I have seen some trucks do it on their 24v system, by grabbing the 12v from the rear battery in the bank, it partially compensates for the the front battery not charging as much (due to wind chill effect, the front battery in the pack will often be slower to charge)
eta
you may want to look at a 12/24v changover switch, hmm maybe not, they were used commonly in trucks where they had a 12v alternator and lights etc, but a 24v starter Why the 24v alternator???
I think the loads needing 12v, as suggested before, use a 12v converter If they are lighting- many leds are 12/24v... Edited by Boppa 2018-07-27
Chopperp Guru Joined: 03/01/2018 Location: AustraliaPosts: 1057
Posted: 10:46am 26 Jul 2018
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The 20HP diesel engine drives a 24V, 150A alternator plus 2 compressors for two air conditioning systems in a Railcar. The idea was that the alternator would drive the 24V electrics (mainly fans but also the compressor clutches) without the use of batteries.
When the engine was fired up, the alternator gave no output. I had little to do with the system at this stage, just being a casual onlooker. The alternator is a brushless self exciting one with an integrated regulator so I think it was assumed that no batteries were required to get it going.
I got involved & concluded that it did need some volts into it to get it fired up. With 17V or greater from a power supply connected to the 24V output (no load), brought it to life & it was quite happy outputting 28A @ over 30A when some load was connected. Two 12V 7AH cell cells with a series limiting resistor also worked ok.
I then connected a 12V - 24V boost converter to the 12V start battery with the output going to the 24V line. That also worked & was deemed by some of us to be the easiest & cheapest solution. But another thought that we could use two 12V batteries to satisfy the starting requirements of the alternator & also use one of the 12V batteries to start & run the engine electrics. He was worried about the reliability of the converter module. (We'll have spares)
It took a bit of explaining why this was not a good idea & he eventually agreed. We will run with the boost converter idea as a trial.
That's it in a nutshell (Well, maybe a coconut shell) ChopperP
Chopperp Guru Joined: 03/01/2018 Location: AustraliaPosts: 1057
Posted: 10:54am 26 Jul 2018
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In some ways I agree. However, we do not want the cost of additional batteries & they will be mounted under a Railcar & changing batteries around would be awkward & would need monitoring.
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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