I have a very simple system with a 420 watt solar panel, a Victron mppt 75/15 and two AGM batteries (100 amp each) in series to give me 24 volts. There is also a 12 volt converter that runs a diesel heater as emergency heating backup.
The reason for the 24 volt rather than 12 volt is that the diesel heater is very fussy about voltage and needs to go through a shutdown phase when switching off.
I have now ordered a couple of 12 volt 100 amp Lithium batteries to replace the AGM.
I intend to leave them in series for 24 volts, would that be the best thing to do?
What charging settings should I select for the MPPT 75/15?
Should I wish to run a small inverter off the batteries to power my Starlink router in the future, is there any upgrade that I need to make to help?
Thank you.
With 2 12v in series you’ll need a balacer.
Why didn’t you order a 24v battery ?
I didn’t buy one 24 volt battery because it would have been more expensive and as I am new to all of this, thought that I might need to go back to 12 volts at some time in the future.
Perhaps I should not have bothered if I am having to buy even more equipment.
As the lithium is much more stable then the lead batteries, I’d put them in parallel and feed the diesel-unit directly from there.
Btw, you should have used the balancer with the old batteries too
Thank you. Didn’t realise that I should have had a balancer on the old batteries! One is a few years old and one is almost new, but that is done now.
I have ordered a balancer for 24 volt now.
Do you have any idea on the charging selection from the MPPT 75/15?
You can’t do much wrong if you set it to lithium in the dropdown list
As batteries wont arrive at the same state of charge, it would be a good idea to fully charge each battery before wiring in series.
Thank you for that. The Lithiums are both coming together from the same retailer, so I had assumed that they would be the same state. I will check, but have ordered a balancer now anyway.
I’d expect them to be within 100mv.
If you can, charge them as 12v in parallel and leave them cycling in the higher SOC with a small load to let the cell balancers do their job. If you do run them dead flat and one of the BMS shuts off the polarity will reverse and you will get close to zero volts. When this does happen you will need to disconnect the load and wake the dead battery up with a 12v charger. It’s an interesting phenomenon the first time you see it.
Sounds frightening! 
Not at all just something that happens. Low voltage protection is not as frightening as overcurrent and overvoltage protection. That is when things break.