robert-van-rooyen avatar image
robert-van-rooyen asked

Lithium charge cycle into lead acid battery

I have two questions relating to charging a lead acid battery with a lithium charge profile. On the vessel there will be 1x lead acid starting battery and 1x lithium house battery. The alternator will be upgraded to a Balmar unit with its own external regulator, programmed to a lithium charge profile. There will also be a Victron blue smart charger for shore power charging, I will set this charger to the lithium profile too.

This means the lithium house battery will be getting an optimized lithium charge from the alternator as well as the shore power charger, but the lead acid will also be getting this same profile. Is this okay for the lead acid battery and what is the potential disadvantages to the lead acid?

Relating to the configuration mentioned above, with the alternator charging, I assume a splitting diode for the charge is the best option rather than a charge relay?

I want to keep the system as simple as possible with the least amount of components and avoid dc-dc chargers as I want to utilize the full amps the alternator can give.

Lithium Batteryalternatorbluesmart ip22lead-acidbalmar
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2 Answers
Trevor Bird avatar image
Trevor Bird answered ·

Hello @Robert van Rooyen . The system sounds good. The fundamental difference between the charge profile is the voltage setting of the Absorption and Float phase of the charger. Other issues such as changing the charge voltages in accordance with temperature are not required with Lithium but are advised for AGM. The end result of subjecting the AGM battery to higher than recommended voltages will shorten the life of the AGM.

The best way to split the alternator to charge different battery banks ( start and house) is to use a FET splitter. Diodes area bit old fashioned now because they introduce a small but significant forward voltage drop (0.7 volts). FET splitters do not have that forward voltage drop.

While I understand you want a simple solution to charging of the start and house batteries they have fundamentally different jobs to do. The start battery uses a short burst of energy to start the engine and does not take much time to recharge back to where it started from.

The house bank is drained overnight and during high use and unlike the start battery, needs a lot of current for a long period of time to replenish the charge.

The use of a Dc-DC charger allows the start battery to be charged with recommended parameters and is perfectly capable of replenishing the very small amount of energy required to fully charge the start battery.

I hope this helps.


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robert-van-rooyen avatar image robert-van-rooyen commented ·

Hi @Trevor Bird

thanks for your reply! Okay, understood that a FET splitter will do a more efficient job than a diode splitter.

I forgot to mention that with this yacht, the engine is running 100% of the time as the engine drives a PTO pump which is used 100% of the time while sailing.

The only reason for the lithium battery instead of lead acid is weight and the Balmar set up for reliability. There is no real discharge of the house bank, only the short times of disconnecting shore power before staring the engine (15mins for example)

I am not sure any of this additional info changes anything from my original post?

If I were to keep the system as per my original post but using a FET splitter, the only downfall would be lifespan on the AGM start battery?

If I were to then add a dc-dc charger to charge the AGM battery, giving the AGM a proper charge profile, would I connect the shore power charger (set to a lithium ion charge profile) on the same line as the Balmar regulators output, running through the dc-dc charger as per my diagram (this means the dc-dc charger will be working 24/7, but won't ever really be loaded)? And if it is done this way, is there any risk in a situation when the Balmar system is charging (engine on) together with the shore power charger (shore power still plugged in). Potential risk/damage to the Balmar regulator and/or the Victron shore power charger, or is there another way I should input the shore power charger, considering the charger would be set to a lithium charge profile and has 3 outputs (Victron Blue Smart 12-30)

Sorry for the long explanation, any advice will be appreciated.



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Trevor Bird avatar image
Trevor Bird answered ·

Hello @Robert van Rooyen , I think your drawing is good the way it is. That must be a very large yacht to have engine running for the PTO, presumably to run sheet winches.

You can parallel charging sources to the battery without concern.

You do need to ensure the Balmar external regulator has the alternator temperature sensor connected so the regulator derates the field current of the alternator if the alternator heats up. The Lithiums will draw a lot of current for a long time if they are discharged and that causes most alternators some concern if temperature sensing is not employed. This is important and should not be ignored.

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