Lithium as Backup and DC to DC charger info needed

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ako asked Jul 3, '23

Lithium as Backup and DC to DC charger info needed

I am hoping someone can help me with some questions on the Victron DC to DC chargers and the suitability of Lithium batteries as a backup . I would like to buy a 100am LifePO4 and leave constantly on charge ready as a backup but need to confirm some aspects of the setup first.

What model Victron DC/DC do I need , I want to charge 24 volt lithium through a charger drawing power from my systems 24 volt Bus Bar , i cant see a 24-24 in the range but some say suitable for 24 volt

What is difference between isolated or non isolated

So many makes of batteries , other than Victron and Renology I haven't heard of any of them so how to know what batteries are good or not so good other than the reviews which I know from experience cant be trusted

Might be more convenient physical to use 2x12 volt batteries linked together in series than 1x24 volt , are there any disadvantages to doing it that way .

Can they be left connected permanently without damaging Lithium batteries or Charger

I contacted a company selling batteries on Amazon Spain called Power Queen EU to say I wanted to use one of their batteries as a backup so would get very little use and will go long periods with no use at all , I asked if their batteries can be left on a Victron DC to DC charger permanently so they will be always at 100% and ready for when they are needed , they said “ Normally, we suggest installing a switch between the MPPT and the battery; after the battery is fully charged “

Many thangs in advance to anyone who can assist

charger

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mvas answered · Jul 03, 2023 at 04:42 PM

Technically, we do not leave LiFePO4 "constantly on charge". But a good LiFePO4 charger will maintain the proper level over time.
Isolated = no connection between input negative wire and output negative wire.
I would keep the the battery at no higher than 90% SOC float. You can adjust the float voltage for max life vs max energy.

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Answer 1 · 2023-07-04T15:30:17Z

ako answered · Jul 04, 2023 at 03:30 PM

Thank you mvas , i have now ordered a Victron non isolated 24-24 DC to DC Charger . The only question i have now is why do you suggest keeping batteries no higher then 90% , when they are required it will be for backup so i would have thought 100% would be the logical level to keep them at , is there a disadvantage to keeping them lower . I hoped a Victron Charger would protect them as its a quality product and recommended by so many .

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kevgermany ♦♦ commented · Jul 04, 2023 at 09:12 PM

It's important to charge the batteries to 100% reasonably often to enable the BMS to balance the cells. Rule of thumb is that balancing doesn't start until 90% SOC, but it's not universal. If the batteries really are standby only, self discharge is so low that you could do this quarterly.

You might as well disconnect the charger and simply charge once every three months, there's no need for a trickle charge, unlike lead based batteries.

Maybe think about AGM, in standby use, they last many years, the expense of LiFePO4 is not really justified. These do need constant charging.

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mvas kevgermany ♦♦ commented · Jul 04, 2023 at 09:28 PM

There is no reason to disconnect a LiFePO4 battery pack that has been sitting in float, just to balance the cells. The cells will be balanced the next time the battery goes through a discharge / charge cycle.

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mvas commented · Jul 04, 2023 at 09:22 PM

Because you are designing a UPS backup type system you do NOT want to keep the batteries at 100% SOC during float. That will shorten their life. As I stated in my previous message it is a trade-off of max life vs max energy.

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