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Old 09-21-2016, 07:39 AM   #1
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Using 24v to get better 12v load performance?

DISCLAIMER: I'm not an electrician or an electrical engineer.

QUESTION: Can I somehow "regulate" a 24v battery bank to produce constant 12v (while keeping the bank in a 24v configuration) even under load?

BACKGROUND: In in our renovated AS we have a 4x6v battery bank wired in series-parallel for 12v. I am finding that the components I have on the 12v system are too much of a draw. For example, with 3 Fantastic Ceiling Fans going at the same time (and maybe a few other things like LED lights) the 12v system is drawn down to the 10v range which triggers low-voltage alarms on the fridge, inverter and other devices. I would just ignore the low voltage alarms but the beeping drives me crazy and I can't turn it off!

My thought was that maybe there is a device that could be between a 24v battery bank (my 6v's re-wired in series) and the 12v system that could just "add" voltage to the 12v system (from the 24v bank) as needed to maintain a set voltage whatever the load.

I think what I'm looking for is a "voltage regulator" but I've never heard or someone using one in an RV application.

Is this a bad idea for some reason?
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Old 09-21-2016, 08:18 AM   #2
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This may be a moot point as I plan to swap my current battery bank for Lithium in the near future, but the discussion is one from which I'd still benefit.
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Old 09-21-2016, 08:27 AM   #3
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Is this a bad idea for some reason?
It's a valid idea but with a 4x6 battery setup (presumably the 6v batteries are each pretty large?) you should not have a problem maintaining 12 volts. When your batteries are run down to 10 volts, they are down below about 10% charge, and you're doing them permanent damage. Lead acid batteries should not be discharged below 50% of their capacity for best life.

If you don't have one, get something like the Bogart Tri-Metric battery monitor and, after you've fully charged your batteries, measure the amp-hours that your batteries can deliver, until the system is at 12 volts. That will tell you what the available battery capacity is. You might find that it's far below what the name-plate rating of the batteries should suggest; in that case the batteries are either low on water, not sufficiently charged, and/or extended use at low charge has permanently sulfated the cells and they need to be replaced.
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Old 09-21-2016, 08:29 AM   #4
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This may be a moot point as I plan to swap my current battery bank for Lithium in the near future, but the discussion is one from which I'd still benefit.
If you're planning to go with lithium then you definitely need to be aware of the system state of charge and make sure you have a properly programmed charger.
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Old 09-21-2016, 08:48 AM   #5
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There is no free lunch. Your fans and other loads require the same amount of energy whether the voltage is 12 or 24. The current is lower at 24v though. If you have taken enough energy out of the batteries to drop voltage that much, you won't change that by running the batteries in series.
But, there are dc-dc converters that take 24v and output regulated 12v although I do not see why you would want that. The advantage to your 6v batteries is that they have more amp-hour capacity than standard 12v batteries so you have something squirrely with your batteries if you have been fully charging them.
You will have a host of other issues with a Lithium system. I have just bought the hardware for the conversion and am noodling my way through that now.

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Old 09-21-2016, 08:59 AM   #6
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When your batteries are run down to 10 volts, they are down below about 10% charge, and you're doing them permanent damage. Lead acid batteries should not be discharged below 50% of their capacity for best life.
Sorry, should have been clearer... my batteries are at 100% state of charge in this scenario... For example, after I charge them with no load on the 12v system they stable voltage is around 12.8. Immediately after I turn on the fans, the system voltage drops down to something like 10.8 due to the draw of each component.

I have a Victron BMS that monitors Amps in/out to give me Percent state of charge. I can run these fans all day and night and it takes about 10% out of the battery bank every 8 hours or so.
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Old 09-21-2016, 09:00 AM   #7
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Even the reducer will use power, I am going to watch this...
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Old 09-21-2016, 09:00 AM   #8
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There is no free lunch. Your fans and other loads require the same amount of energy whether the voltage is 12 or 24.
I get that amps in/out don't change if you switch from 12v to 24v, I'm just talking about maintaining a higher current (i.e. 12.8v) even under load to avoid the low voltage alarms going off.

Am I missing something? Ready and waiting to learn...
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Old 09-21-2016, 09:03 AM   #9
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If the little bit of load you report (i.e., three FF and a few LEDs) brings your indicated voltage down to 10 volts while under load (from presumably 12.5 to 12.7 volts resting,) your batteries themselves are likely essentially shot. With reasonably fully charged batteries, you should be able to run those devices and show a minimal voltage drop on your volt meter from the reading you get at a resting state.

Note, In my comment above, I'm not talking about reading 10 volts from your batteries in a resting state and I assume you are not as well, but rather what your voltmeter shows while the batteries are under load.
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Old 09-21-2016, 09:12 AM   #10
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If the little bit of load you report (i.e., three FF and a few LEDs) brings your indicated voltage down to 10 volts while under load (from presumably 12.5 to 12.7 volts resting,) your batteries themselves are likely essentially shot. With reasonably fully charged batteries, you should be able to run those devices and show a minimal voltage drop on your volt meter from the reading you get at a resting state.
Argh, I suspected this could be the case as well (batteries shot). Had another case where the voltage drop was unexpectedly high too (see this thread). How can I confirm this? Using a hydrometer? These batteries are only 2 years old; they've been kept at full charge since I've had them (except once).
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Old 09-21-2016, 09:48 AM   #11
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If you switch to lithiums you won't have voltage drops. We have 400ah of lithiums, I can have both fans going, my computer and monitor running and still run the blender, with no dip in voltage.

This is just another reason why I skipped lead acid.

Would 24V handle loads better? Yes. But then you have to down regulate everything, so what's the point of all that effort.
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Old 09-21-2016, 10:34 AM   #12
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Normally the only way you are going to be able to check battery parameters is when the battery has been resting a while. Whether you are checking voltage or specific gravity, parameters available refer to "at rest". Specific gravities are good if you have wet cells because you can check each cell and verify that they are essentially identical. Any gross variation with a cell would clearly point to a bad battery.
But, you have a lot of capacity and the kind of voltage drop you are seeing, if it is at rest, indicates bad batteries. It is meaningless to look at voltage under load. A battery shop can do a load test on your batteries, which is a way to confirm good or bad, but you have to drag the batteries over to the shop.
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Old 09-21-2016, 11:00 AM   #13
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To me it also sounds like you have dead batteries. Get yourself a simple load tester. You can generally find these at any auto parts store or walmarts

https://www.walmart.com/ip/Schumache...ester/21642133
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Old 09-21-2016, 11:51 AM   #14
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To me it also sounds like you have dead batteries. Get yourself a simple load tester. You can generally find these at any auto parts store or walmarts

https://www.walmart.com/ip/Schumache...ester/21642133
Just ordered one. Thanks! I'll report back with results.
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Old 09-21-2016, 12:25 PM   #15
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...the 12v system is drawn down to the 10v range which triggers low-voltage alarms ...?
Are you measuring the 10V at the battery posts or on the display monitor? The voltage drop might be thru the wiring.

I know that one of my circuits has multiple high current loads AND the tank/battery monitor display. If I turn on all of the devices, the tank display shows 10.5V...BUT the Trimetric that is wired directly to the batteries shows 11.8 V

Wiring voltage drop.
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Old 09-21-2016, 12:36 PM   #16
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Are you measuring the 10V at the battery posts or on the display monitor?
It's being measured from the BMS, but the BMS is effectively wired directly to the battery bank, so I don't think there'd be much of a voltage drop from the wiring...
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Old 09-21-2016, 01:21 PM   #17
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QUESTION: Can I somehow "regulate" a 24v battery bank to produce constant 12v (while keeping the bank in a 24v configuration) even under load?
There are switching power supplies available that will do that, yes. Commercial products are sold for that for a couple of related applications. One us the use of 12 volt accessories (such as car audio) with the 24 volt electrical systems used in some military surplus vehicles. There are similar applications in aviation, and in marine systems.

Quote:

BACKGROUND: In in our renovated AS we have a 4x6v battery bank wired in series-parallel for 12v. I am finding that the components I have on the 12v system are too much of a draw. For example, with 3 Fantastic Ceiling Fans going at the same time (and maybe a few other things like LED lights) the 12v system is drawn down to the 10v range which triggers low-voltage alarms on the fridge, inverter and other devices. I would just ignore the low voltage alarms but the beeping drives me crazy and I can't turn it off!
That is because there is something wrong with your setup.

The battery bank itself, if sufficiently charged and working properly, should be able to deliver several hundred amps without dropping below 12v.

There are any number of possible causes. It isn't possible to diagnose them accurately or safely over the internet.

Quote:
Is this a bad idea for some reason?
It's relatively easy for someone with an electrical engineering background to put together a device that will turn a 24 volt input into a 12 volt output. It takes some work (and parts cost) to make a device that will do it efficiently across a wide range of power draws. In round numbers these devices end up being around 90% efficient in the real world. They are also expensive to build, with costs similar to a 120v inverter of similar watt capacity.

You still have the problem of charging the battery, of course, so you would have to replace your 12v converter-charger with a 24v one.

What you suggest is a bad idea because it will not solve your problem, unless by happy circumstance you end up replacing whatever component is failing. A 12v system that isn't working still won't work if it's reconfigured for 24 volts.
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Old 09-21-2016, 03:05 PM   #18
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There are such devices but it won't solve your problem. Your problem is most likely not enough amp-hours of storage capacity and that will be the same at 24V as it is at 12V. Then you would have some loss in the 24V to 12V converter so you would have even less usable storage. As lead acid batteries age they lose storage capacity and behave like smaller batteries and this might be your problem. Good luck with finding a solution and let the forum know what works!
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Old 09-21-2016, 03:05 PM   #19
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In addition, if you hook up four 6V batteries in series to get 24V and one goes bad, you're going to have no power.

If you instead hook those four 6V batteries in series/parallel to get 12V and one goes bad, you can limp along on the good pair. Same if you use a pair of 12V batteries connected in parallel. A bit more flexibility, no voltage converter to build/buy, and so on.

Life's better at 12V when everything around the batteries is built on the assumption of 12V.
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Old 09-26-2016, 05:59 PM   #20
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Does not your battery monitor measure energy? The units would likely be in Whr. You can convert the energy delivered into how much stored energy is in you battery bank. That will tell you how many AH your battery bank can deliver.

To me that kind of voltage drop sounds like a bad electrical connection, those commonly occur at the battery, but could also be at your fuse box. You can search with your volt meter to find a voltage drop across the bad connection.
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