Charging 2x12V batteries connected in series

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rhills
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Charging 2x12V batteries connected in series

Post by rhills » Thu, 04 Oct 2018, 15:44

Hi,

I am looking to repurpose a couple of 100AH 12V AGM batteries as a UPS for our office. I already have a good 24V inverter and a dead 240->24V charger so I need to replace the charger.

240->24V chargers seem to be way more expensive than their 12V counterparts which I am guessing is at least partly because far more of the latter are sold.

I wondered if it is possible to have the batteries connected in series, but have two 240->12V chargers, one connected to each battery. As I see it, each charger is only going to see the potential difference between its battery's terminals, ie 11-14V depending on SOC so I believe it should work. However, I've never seen it done, so I am guessing there's some reason it wouldn't work.

Thoughts?
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jonescg
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Re: Charging 2x12V batteries connected in series

Post by jonescg » Thu, 04 Oct 2018, 16:14

Yep - no problem with that. Just make sure the negative of the charger output is isolated from the earth pin of the charger.
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neilg
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Re: Charging 2x12V batteries connected in series

Post by neilg » Thu, 04 Oct 2018, 22:09

Please be aware that some UPS connect mains neutral to the battery negative so the batteries should be fully enclosed for safety.
neilg.

rhills
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Re: Charging 2x12V batteries connected in series

Post by rhills » Fri, 05 Oct 2018, 01:31

Hi Neil,
neilg wrote:
Thu, 04 Oct 2018, 22:09
Please be aware that some UPS connect mains neutral to the battery negative so the batteries should be fully enclosed for safety.
Thanks for that info. My system is basically Batteries + 240V charger + inverter so it's not a formal UPS as such.

Cheers,
Rob Hills
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Richo
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Re: Charging 2x12V batteries connected in series

Post by Richo » Fri, 05 Oct 2018, 12:19

What is the power rating of the inverter or what is the average load power connected to the battery?
So the short answer is NO but the long answer is YES.
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Re: Charging 2x12V batteries connected in series

Post by T1 Terry » Mon, 08 Oct 2018, 14:31

rhills wrote:
Thu, 04 Oct 2018, 15:44
Hi,

I am looking to repurpose a couple of 100AH 12V AGM batteries as a UPS for our office. I already have a good 24V inverter and a dead 240->24V charger so I need to replace the charger.

240->24V chargers seem to be way more expensive than their 12V counterparts which I am guessing is at least partly because far more of the latter are sold.

I wondered if it is possible to have the batteries connected in series, but have two 240->12V chargers, one connected to each battery. As I see it, each charger is only going to see the potential difference between its battery's terminals, ie 11-14V depending on SOC so I believe it should work. However, I've never seen it done, so I am guessing there's some reason it wouldn't work.

Thoughts?
This method will actually work better than a 24v charger because each battery will charge to 14.4v roughly and then settle back to 13.8v. With a 24v charger the total voltage is 28.8v and back to 27.6v, sounds the same but only if both batteries reach their upper voltage at the same time. One battery at 13.7v and the other at 15.1v also adds up to 28.8v, but not two batteries with the same state of charge.

T1 Terry
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