Two Lithium battery banks for sail boat

DIY
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1

Hello Victron community

I am seeking some inputs, help and clarification regarding lithium battery charging for the new energy system on my sailboat.

I am replacing the old and very little electric system to transform a day cruiser to a long distance offshore sailboat.

I am installing 2x Lithium banks. One for the main house loads and one as a backup for critical (navigation) equipment. The main house bank are 2x LFP Smart 25,6/200-a batteries with a Lynx BMS 500 wich connects them to another Lynx distributor. From this distributor the normal load and the critical loads are separately supplied. The critical loads are connected to the Lynx system via an ‘Automatic Charging Relais’ (BlueSea) and another battery bank of 2x parallel 25.6V 50Ah LiFePO4 Superpack (internal BMS). If the main battery bank fails and there is no charge from Solar panels or alternator., the automatic charging relay will disconnect and the normal load can not drain the navigation load battery bank which in this set up still supplies power to the critical loads. It seems to me to be a good system with redundancy if the main house battery fail and I have seen this done with AGM batteries. Otherwise if the Lynx BMS has an issue and disconnects the Smart batteries, the full boat would be blacked out. The Superpack batteries are a back up for critical navigation loads and could get the boat back to a port. I thought the Superpacks are a slim option with integrated BMS which don’t need another Lynx BMS or other external BMS systems. There is also an Victron EasySolar II GX inverter/charger unit installed, an extra MPPT and a regulated ‘high output alternator’.

WhatsApp Image 2025-08-02 at 20.20.48
WhatsApp Image 2025-08-02 at 20.20.481600×1200 107 KB

Now comes the actual question where my knowledge leaks and I would like to have some opinions:

With the Smart battery/Lynx BMS and the LiFePO4 Superpacks I have two battery systems which are connected via a Automatic Charging Relay in normal operation. Is it possible two charge both battery banks of different kind and maybe different state of charges with one charging source (Easy Solar charging or Zeus regulated alternator)? How good is this for the batteries when one bank requires ‘bulk’ charge and the other ‘float’. Victron seems to make a big story of five or even 7-stage adaptive charging algorithm and I am worried that the batteries would take damage when they require different charging states at the same time. I believe the Lynx BMS dictates to the EasySolar what charging he requires for the Smart batteries but would this also work for the Superpack batteries? Do I just need to make sure both battery banks have the same charge when taking the system in use? I am not sure if I make a too big story out of this? (If so I am sorry :slight_smile: )

I do have already the Smart batteries, Lynx system and the EasySolar II GX. So I could still get another navigation battery system if Victron Superpack LiFePO4 don’t work.

Thank you for your supports and inputs.

Roman

2

reach out to your Victron dealer, they will be able to help you with this and system design in general

3

This algorithm is for lead batteries. LiFePO4 are fine with a CC/CV charge algorithm, and if toy have a ‘float’ stage too, then that is fine. float should be ~100mV less than absorption voltage or as specified by the manufacturer.
Yes, 2 different battery manufacturer’s cells can be charged by one dual output charger provided that 1) they have identical voltage requirements (bulk, float) or 2) the charger is set to the lower of the 2 voltages. Lithium batteries are not harmed by under charging as other battery chemistries are, all that happens is you loose a little bit of capacity.
For long distance sailing: ensure that you have system redundancy, including 3 separate sources for charging the house bank. On our yacht, the 24V house bank also trickle charged the radio and engine batteries. We also had wind, generator and solar as standard charging sources.

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Eventually I do so but I don’t want to go there with empty hands :slight_smile:

5

Hi Mike,
Thank you for your input regarding the charge algorithm. In general, Victron has so many different chargers and to be honest I am a bit confused what the differences are. I am still trying to figure out what the best charging option is for the starter battery and anchor winch battery which are both 12V (AGM). Probably a 24/12 DC DC charger close to the starter battery and another 12/12 DC DC charger near the bow to charge the anchor winch battery to compensate voltage drops from the long cables. I am also preparing the boat for 120VAC from the states which won’t be possible with a EasySolar (MultiPlus) and a Centaur is probably needed that can handle the voltage and frequency.
I am drawing right now the system more nicely and will share it here if you are keen to give it a look :slight_smile:

Thanks, Roman

6

We used a float charger only to maintain the engine start and the radio battery. Engine was retrofitted with a 24V alternator - to suit the house banks, so engine start battery only had the float charger. For dockside charging, you could use a 3 stage or 5 stage charge algorithm, Victron’s default for AGM works fine.
Re any conversion for 120V: Get an isolating transformer that has a 120/240V input. The output voltage from the transformer can be 240V. You then don’t need to change anything else on the wiring system or generator: all 240V equipment will work well enough from 240V 60Hz.
Most marinas should be able to offer 16A/240V outlets as well as the 120V, but get the tapped transformer just in case - we never went to mainland USA due to visa considerations.