mr-frankie avatar image
mr-frankie asked

Lithium/Lead Acid battery bank

Hi, I have a narrowboat with 4 x 110ah lead acid batteries monitored by a BMV 702 and connected to a MPPT 100/50 Victron controller and a Multiplus 12/3000/120 Charger/Inverter.

I am looking to optimise the performance and life of the batteries and came across the combination of Lead Acid and Lithium Batteries in parallel which appear to complement their charactistics for optimum performance and Lead Acid battery life.

It appears that the recharge for the batteries is where this system falls down. Am I correct?

If this is the case, has anyone tried to separate the batteries during charging then reconnect for discharge? Would this overcome the problems in charging?

Your advice gratefully recieved


Lithium Batterylead-acid
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2 Answers
alan avatar image
alan answered ·

To do this you need a Li battery with a (dumb) BMS that simply connects and disconnects when the voltage is outside therange of the lithium. One that doesn't communicate with the venus/victron. You could still use a BMS with can output etc, but at present can't be connected to the victron gear as it only supports a single battery bank.

You set the victron gear with the charge setting of the Lead bank and it's not aware of the Lithium. Simply add the capacity to whatever monitor you use for SOC.

It is a great idea, makes sense. There's more info here.

It would be great to see victron implement the ability to add, and monitor, say a pylontech or similar as a Hybid setup. It would make a transition to lithium from an aging Lead Acid bank a lot easier on the wallet.
Maybe @mvader (Victron Energy) could give his thoughts?

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Hi @Alan

Although some people have made it work they say, in practice it needs careful tuning and monitoring.

Biggest problem is charge characteristics, both type work different, and -very- important: Lead Acid needs temperature compensation.

While discharging the Lithium battery does -all- the work until it is empty, only then the Lead Acids do some work.

So...not practical, and a support nightmare for anyone who want's to implement it.

Please keep in mind that a 'dumb' lithium battery with internal disconnecting BMS also is not supposed to be discharged up to the disconnect voltage, that is to be seen as a last safety resort.

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alan avatar image alan Daniël Boekel (Victron Energy Staff) ♦♦ commented ·

Yes I agree that it needs to be carefully monitored, which is why the best way would be to implement monitoring to it.

I guess dumb isn't the best term, a BMS with a settable LVD and HVD is required. Or alternatively one controlled by a system monitoring device "hint, hint" ;-). That can monitor the SOC of the lithium and disconnect and reconnect it between 10-90%

As for temp compensation, the voltage adjustment would fall outside the low volt range of lithium at high temps, but if the BMS has disconnected them, they just won't be charged, won't be damaged by being left discharged and can charge again once the temp falls.

Pylontech is probably a good example as they don't use a standard setup and decided to go for a 15s pack. So the voltage is already lower.

This would work very well and allow for Temp compensation of the lead acid during charge, as soon as load is applied the lead voltage would drop below the lithium and allow it to discharge first.

Lead-acid batteries

  • Hate to cycle
  • Love to remain fully charged
  • Hate being left discharged

Lithium batteries

  • Love to cycle
  • Hate to remain fully charged
  • Fine with being left discharged

You want the lithium to discharge first, then ideally recharge last, however they will also recharge first due to the voltage window unless controlled by the system. You could keep the CCL to 0A until the lead acid are in absorption. Then let the extra\spare current available charge the lithium. Maximising input, especially off a genset.

The way this can work well is by working to the strengths of both batteries, the lithium won't care if its left discharged and hates being kept on float.

1 Like 1 ·
mr-frankie avatar image mr-frankie Daniël Boekel (Victron Energy Staff) ♦♦ commented ·

Hi, I'm sort of following the technical stuff, to clarify, it's the recharge which is the issue having a recharge compatible with the battery technology?

To overcome this, if there was a disconnection relay that operates when recharge starts (engine running) that separates the two battery technologies and a DC lithium charger connected to the lead acid battery system. Would this overcome the problems in charging or is there more incompatibilities that have to be addressed?

thanks Graham

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Paul Carmichael avatar image Paul Carmichael commented ·
Hello all. I have a bank of lead/acid 910ah batteries. After 6 years they're showing signs of age. If we let the charge drop below 80% at night, any load will drag the voltage down below 40, which sets off alarms, shortly followed by a system shutdown. I realise one day I'll have to bite the bullet and buy lithium, but a) I'm not welathy and b) I don't really want tto throw away all the existing cells (they cost me 8000€). So the possibility of something in parallel appeals. I read this thread, but to be honest, although I'm a fairly technical person, I got a bit lost in terminology etc. I'm now going to follow the link above, but if anyone could advise, I'd love to hear it.
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pwfarnell avatar image
pwfarnell answered ·


It is worth noting that ISO/TS 23625 Small craft — Lithium-ion batteries states that different battery chemistries should not be mixed even with auto combiners. I am not sure that this applies to narrow boats in th UK, it may for new builds.I

There is a UK Facebook group that advocates this adding a long wire in the system as a voltage reduction. The other issue is they rely on the BMS to disconnect the charge, so the safety system is also acting as the control system. I always consider you need a control system to properly charge the batteries and an independent safety system to isolate them if something goes wrong.

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