Charger for 800Ah battery bank

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hookem asked May 25, '21

Charger for 800Ah battery bank

I'm planning to install a Mini-Split on a motorhome, with a dedicated 800Ah Lithium battery bank and a 230V Inverter (Phoenix Inverter Smart 1600VA). For batteries, I'm leaning towards (4 * 12.8/200Ah), for less weight and the option to set the bank at 12V or 24V.
So my questions are:
1) Should I set the bank at 12V or 24V (I'm leaning towards 24V for the option of connecting a DC-DC converter to charge my 12V the "House" batteries: (2 * 125Ah), Lithium.
2) Which Charger should I consider when connected to Shore/Gen power (120V/30A service).?

charger

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Answer 1 · 2021-05-25T02:24:29Z

Vance Mitchell answered · May 25, 2021 at 02:24 AM

If you upgrade from the Phoenix to the multiplus it will have the charger you require.

The built in charger in the 1600va units is either 70A(12v) or 40A(24v) which should be ideal for that battery bank (c/10 charge rate).

Personally I would go 24v but how the rest of your system is configured could change that.

Double check the starting current of your split system, some units require more to start than a 1600va inverter can handle and it’s always nice to have a bit of headroom in case you add devices later.

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hookem commented · May 25, 2021 at 02:42 AM

I don't think I can use a MultiPlus.
In my case, the Inverter is 230VAC but the Charger would work from 120VAC (Shore power).,,, I think Mutiplus works with the same voltage on the AC side (e.g., 230V).

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Vance Mitchell hookem commented · May 25, 2021 at 04:53 AM

Sorry I missed the 120VAC requirement.

Surprisingly my recommendation still partially stands, A 120V Multiplus is close enough in price that it's additional features could be worth considering.

The only downside to using a multiplus as the main charger is that it can only be used at the specified input voltage whereas a skylla charger has a wide input range allowing it to be used from 120VAC or 240VAC.

The Skylla IP65 Charger would work well with your battery bank.

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hookem Vance Mitchell commented · May 25, 2021 at 08:35 PM

Thanks... Since I'm trying to build an 800 Ah bank, my concern about the Skylla IP65 Charger is that its spec says:
Battery capacity 350-700 Ah @ 12V, and 150-350 Ah @ 24V

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Vance Mitchell hookem commented · May 26, 2021 at 02:21 AM

It is normally recommended to charge lead acid batteries at around a C/10 rate.

Lithium can generally accept a higher charge rate but the IP65 charger would still charge your battery in around 12 hours if it was flat when you started charging.

Do you plan to add solar or any other charge source to your system?

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hookem Vance Mitchell commented · May 26, 2021 at 07:42 PM

Yes, 1600W of Solar,

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Answer 2 · 2021-05-26T04:05:05Z

Matthias Lange - DE answered · May 26, 2021 at 04:05 AM

A lead/Gel/AGM battery should be changed with a current based on the capacity but for lithium batteries it's not that important.

800Ah lead/Gel/AGM should be changed with 80-120A for 10-12h.

800Ah lithium you can change with 10A but this will take e few days. But you also can change it (depending on what the datasheet says) with 400A or more.

e.g. Victron recommends less than 1C but you can charge with up to 2C -> 800-1600A!

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Warwick Bruce Chapman commented · May 27, 2021 at 08:35 PM

A few weeks ago, I was trying to find some science to back up the contention you've made here about lead charging. The rule of thumb I learned with lead was 10-15% of C as charger current. I have an inherited site with 680Ah of lead on a 35A MultiPlus and I advised the client to get a bigger charger and related the rule of thumb but when asked for more detail as to exactly why, I was unable to substantiate or later find anything definitive online. Why does lead require 10-15% (or whatever) of C for charging?

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Matthias Lange - DE ♦ Warwick Bruce Chapman commented · May 27, 2021 at 08:44 PM

If the current is to high the voltage rises to fast and reaches the absorption voltage to fast -> the charger "thinks" the battery is full and switches faster from bulk to absorption to float but the battery isn't charged correctly.

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Warwick Bruce Chapman Matthias Lange - DE ♦ commented · May 27, 2021 at 08:48 PM

Yes, but I'm asking about having a charger where the current is too low. In this case C/20.

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Matthias Lange - DE ♦ Warwick Bruce Chapman commented · May 27, 2021 at 09:07 PM

To be honest I'm not sure about that.

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Vance Mitchell Warwick Bruce Chapman commented · May 27, 2021 at 10:59 PM

'The switch from Stage 1 to 2 occurs seamlessly and happens when the battery reaches the set voltage limit. The current begins to drop as the battery starts to saturate; full charge is reached when the current decreases to 3–5 percent of the Ah rating. A battery with high leakage may never attain this low saturation current, and a plateau timer takes over to end the charge.'

Quoted from Battery University

I take that to mean that if you are using a charger with less than C/20 capability that the battery might not reach full charge due to losses as a lead acid battery approaches full charge. This also relates to how lead acid charging becomes less efficient as you get closer to full charge.

On the same note if the charger can handle more than the average load it will still be able to maintain the battery at close to full charge, what effect this has on lifespan of the battery would be interesting to know.

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