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Author Topic: 24V system charging from 12V source  (Read 2026 times)

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Lazydocker

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24V system charging from 12V source
« on: 26 October 2019, 08:13:12 »

Right guys. I’m after some confirmation how to do this. Some background - I can’t go into too much detail as this is “top secret”!

Looking to fit something that has a 7.2kW draw at 32A. Ideally I want to run this from a 10kW inverter instead of a generator but this needs a 24V input. Firstly, 2 x 100Ah batteries in series provides the 24V feed and should give 3 hours use... Yes?

By my reckoning this is relatively simple but I need someone to confirm - The batteries can be charged individually with split charge relays connected to each battery but the question is does the series link need to be isolated whenever the vehicle is running?

I thought I had this sussed out but have managed to confuse myself over the last few days, possibly because I’ve started overthinking it!

Failing that it looks like I’ll need to come up with another solution...

Any thoughts or advice gratefully received.
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Kevin Wood

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Re: 24V system charging from 12V source
« Reply #1 on: 26 October 2019, 09:45:10 »

I think you need a bigger battery. 7200 W at 24 V is 7200/24 or 300 Amps! So a 100 Ah battery will power it for 1/3rd of an hour not 3 hours.

This is also in an ideal world where the inverter is 100% efficient and the batteries deliver to their rated capacity without drooping low enough for the inverter to drop out. Given the high discharge current, I think you'd need to go considerably higher than the theoretical capacity to make it work in practice and 300 A of continuous draw from a 100 Ah battery is not trivial. I would expect it to need to be quite a special battery for that to work without it getting exciting. Even vehicle starting batteries only sustain that for a few seconds at a time, and their construction for short term high current use makes them unsuitable for cyclic applications such as this.

If you want to charge the batteries from 12V then yes, you'll need to effectively switch them into a parallel combination and put them across the vehicle alternator. You can't do this without breaking the series link between the batteries.

To be honest, I think a generator would be the best option, and probably much more compact than the batteries that would be required to guarantee operation for 3 hours.

I'm assuming this load draws 7.2 kw constantly, of course. If it's only intermittently that might change things.



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Lazydocker

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Re: 24V system charging from 12V source
« Reply #2 on: 26 October 2019, 10:28:02 »

Thanks Kevin.

It’s a 240V, 32A input. 7.2kW draw that will be plugged into the 10kW continuous (30kW peak) inverter.

We only need to run it for 30-45 minutes and, ideally, still have the engine running to trickle the batteries - but this is now not a simple option as they need to be 24V for the feed and the bridge needs to break for charging. I have seen this: https://www.yandina.com/troll2400info.htm as an option...

The generator option is a last resort as doesn’t fit well with the other equipment fitted - plus there’s a H&S issue with the weight and noise of a generator big enough.

All that said, it’s looking a lot like the generator may be the only real option... I was hoping someone was going to come up with a simple solution!
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Doctor Gollum

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Re: 24V system charging from 12V source
« Reply #3 on: 26 October 2019, 11:54:05 »

AR Peachment do a hydraulically driven generator if you have access to a PTO... Also consider that an engine of some description could be used to do other things such as provide heat/hot water if applicable... ;)
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Marks DTM Calib

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Re: 24V system charging from 12V source
« Reply #4 on: 26 October 2019, 14:40:49 »

So you need 6 off 100 Ah batteries in series/parallel arrangement to get the required capacity/time.

Your also into the levels of bus bars to keep the volt drop to a minimum.

Forget using the charger to support, its likely to do very little under these conditions.

Generator would probably be much easier and safer!
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Lazydocker

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Re: 24V system charging from 12V source
« Reply #5 on: 26 October 2019, 18:59:05 »

So inverter and batteries is a no go really then ::) :D

Thanks guys... I knows I was missing something but couldn’t work out what ::)
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STEMO

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Re: 24V system charging from 12V source
« Reply #6 on: 26 October 2019, 19:55:38 »

So inverter and batteries is a no go really then ::) :D

Thanks guys... I knows I was missing something but couldn’t work out what ::)
A generator.  :)
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Lazydocker

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Re: 24V system charging from 12V source
« Reply #7 on: 26 October 2019, 22:32:28 »

So inverter and batteries is a no go really then ::) :D

Thanks guys... I knows I was missing something but couldn’t work out what ::)
A generator.  :)

Cheers uncle. I knew someone would help :-X ::)
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