lithium and agm batteries: a warning !

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   as many of us , I have service lithium batteries and lead agm batteries for the engine starter and generator.
everything worked fine for three years , up to this morning ...when I try to start the engine and the battery wasn't providing enough current to the starter motor. after few minutes of generator , the engine started and I will replace the battery ASAP...

...but I kept thinking about why it happened, and I think I have an explanation.

yesterday it was very hot and I ran the aircon from batteries. I stopped the aircon around 40% about, but it went down overnight to 12% (usually , I never go below 30%).

in my system , the lithium battery is charged by the same chargers (the masscombi  and the solar panels) charging the agm, and I never thought what this means: they are subject to the same voltages, therefore they are in parallel!

let's forget for a second about the charger , because that's not what was happening last night.

I checked all the relevant voltages of the discharge cycles and we have three configurations.

lfp soc > 15% (temp. dependent). the service battery has a voltage higher than a fully charged agm battery, so it will  trickle charge the agm batteries all the time. the agm batteries therefore never "float", and this reduces the lifetime. I somehow knew it ...but accepted it. this is the normal situation...

SOC around 15%: the voltage of the lfp start going below the fully charged AGM battery , and the agm battery starts charging the lfp battery .. that's 10x bigger , so it's voltage starts dropping and you have an equilibrium for a while...but the agm starts emptying fast ...

SOC below 15%: the lfp voltage drops faster than the almost linear Agm voltage drop , but essentially the AGM battery empties into the Lfp battery than has a larger capacity. now the engine may not start anymore  !!! 

conclusion: I never explored this regime , but it's clearly not good. the agm battery can be fully discharged by the service battery when the  SOC goes  below 15%, and that's not what you want.

solution: I will install either a battery charger (like the mastervolt mac plus 12/12-50) or a converter (like the mastervolt magic 12/12-20) to charge the agm batteries...I prefer mastervolt because all components (batteries , mppt, inverter) are mastervolt and I have a czone monitor .

in both devices  , I can implement floating for the agm batteries , improving their lifetime and avoiding them to be discharged into the Lfp service battery.

lesson learnt , I hope...

addendum: studying the masscombi 12/2200-100 manual I discovered that there is a secondary 5A charging port...but in my opinion the boat schematics says it's not used , and the battery is attached directly to the main 12V main bus . Am I right or wrong ?

Edited by Pzucchel - 04 July 2025 at 11:35

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Pzucchel Members Profile Send Private Message Find Members Posts Add to Buddy List Rear Admiral Joined: 13 March 2020 Location: floating Status: Offline Points: 659	Post Options Post Reply Quote Pzucchel Report Post Thanks(0) Quote Reply Topic: lithium and agm batteries: a warning ! Posted: 04 July 2025 at 10:11
	as many of us , I have service lithium batteries and lead agm batteries for the engine starter and generator. everything worked fine for three years , up to this morning ...when I try to start the engine and the battery wasn't providing enough current to the starter motor. after few minutes of generator , the engine started and I will replace the battery ASAP... ...but I kept thinking about why it happened, and I think I have an explanation. yesterday it was very hot and I ran the aircon from batteries. I stopped the aircon around 40% about, but it went down overnight to 12% (usually , I never go below 30%). in my system , the lithium battery is charged by the same chargers (the masscombi and the solar panels) charging the agm, and I never thought what this means: they are subject to the same voltages, therefore they are in parallel! let's forget for a second about the charger , because that's not what was happening last night. I checked all the relevant voltages of the discharge cycles and we have three configurations. lfp soc > 15% (temp. dependent). the service battery has a voltage higher than a fully charged agm battery, so it will trickle charge the agm batteries all the time. the agm batteries therefore never "float", and this reduces the lifetime. I somehow knew it ...but accepted it. this is the normal situation... SOC around 15%: the voltage of the lfp start going below the fully charged AGM battery , and the agm battery starts charging the lfp battery .. that's 10x bigger , so it's voltage starts dropping and you have an equilibrium for a while...but the agm starts emptying fast ... SOC below 15%: the lfp voltage drops faster than the almost linear Agm voltage drop , but essentially the AGM battery empties into the Lfp battery than has a larger capacity. now the engine may not start anymore !!! conclusion: I never explored this regime , but it's clearly not good. the agm battery can be fully discharged by the service battery when the SOC goes below 15%, and that's not what you want. solution: I will install either a battery charger (like the mastervolt mac plus 12/12-50) or a converter (like the mastervolt magic 12/12-20) to charge the agm batteries...I prefer mastervolt because all components (batteries , mppt, inverter) are mastervolt and I have a czone monitor . in both devices , I can implement floating for the agm batteries , improving their lifetime and avoiding them to be discharged into the Lfp service battery. lesson learnt , I hope... addendum: studying the masscombi 12/2200-100 manual I discovered that there is a secondary 5A charging port...but in my opinion the boat schematics says it's not used , and the battery is attached directly to the main 12V main bus . Am I right or wrong ? Edited by Pzucchel - 04 July 2025 at 11:35
	Hanse588#55

Ian Coverdale Members Profile Send Private Message Find Members Posts Add to Buddy List Captain Joined: 19 June 2019 Location: Travelling Status: Offline Points: 311	Post Options Post Reply Quote Ian Coverdale Report Post Thanks(0) Quote Reply Posted: 04 July 2025 at 11:43
	Very interesting. Paralleling battery chemistries always seemed like a very bad idea to me but there people our there evangelical about paralleling Pb batteries with LiFePO4 to mitigate for BMS going into shutdown when real answer is to have a BMS/monitoring system that alerts user to a potential shutdown condition (as now required by ABYC .. if you like in North America) before the shutdown occurs. Wonder how your Pb/LiFePO4 batteries parallel. Is that through a battery combiner or a function within your charger? Some (most) auto-voltage sensing battery combiners need upgrading to LiFePO4 compatible models otherwise engine/service parallel at too low a voltage. Sounds like DC-DC converter to charge Pb from LiFePO4 is the answer.
	Ian & Andrea SV Gabrielle (H445) Liveaboards - currently Montenegro. www.facebook.com/sailinggabrielle

marsella Members Profile Send Private Message Find Members Posts Add to Buddy List Rear Admiral Joined: 21 June 2022 Location: Antigua Jolly H Status: Offline Points: 592	Post Options Post Reply Quote marsella Report Post Thanks(0) Quote Reply Posted: 04 July 2025 at 11:44
	I like DCDC chargers. Apparently I see that they take care of my thruster batteries very well. I was also thinking to add one for the engine battery, but where would it go because there is an alternator already which if upgraded does 3 stage charging with Li profile, there is a battery combiner and solar, shore power chargers, which also setup for Li.

Pzucchel Members Profile Send Private Message Find Members Posts Add to Buddy List Rear Admiral Joined: 13 March 2020 Location: floating Status: Offline Points: 659	Post Options Post Reply Quote Pzucchel Report Post Thanks(0) Quote Reply Posted: 04 July 2025 at 13:22
	I am also very happy of the 12v/24v converters for the thrusters. I was indeed thinking to install the same converter for the engine starter battery but I need to study carefully the schematics. as I mentioned there is also a secondary charger on the inverter that hanse decided not to use:why? Then , isolating the battery with a dc/dc converter would it mean that the alternator stops charging the starter battery directly at high current ? in that case , is 5A trickle charge enough ?
	Hanse588#55

Arcadia Members Profile Send Private Message Find Members Posts Add to Buddy List Admiral Joined: 27 November 2017 Location: Sag Harbor, USA Status: Offline Points: 1112	Post Options Post Reply Quote Arcadia Report Post Thanks(0) Quote Reply Posted: 04 July 2025 at 14:35
	Piero, it seems that the engine and service batteries on Andrew are wired together and not via a voltage triggered battery combiner as is on the factory Hanse set up. Also, the alternator is wired directly to the engine battery while the service bank would get charged via the combiner. This set up caused problems for me. I moved the alternator directly to the service bank and modified the combiner so it would trigger with any charge voltage instead of the thresholds that didn’t work. So I’m not sure how your engine and service are combined but they should be separated when not being actively charged. A dc to dc between the service and engine bank is best. But be sure to move the alternator charge cable directly to the service bank along with the other charge sources. The 5 amp from the Mastervolt would be OK but would only charge with shore power or generator, and not the alternator. Edited by Arcadia - 04 July 2025 at 14:37
	Leon / ARCADIA 2018 Hanse 588 Sag Harbor, NY

marsella Members Profile Send Private Message Find Members Posts Add to Buddy List Rear Admiral Joined: 21 June 2022 Location: Antigua Jolly H Status: Offline Points: 592	Post Options Post Reply Quote marsella Report Post Thanks(0) Quote Reply Posted: 04 July 2025 at 14:49
	So just to understand, alternator, solar, and all other charging sources go to the service bank from where 3 DCDC charges go to engine, stern and bow batteries. No battery combiner needed in this case?

Arcadia Members Profile Send Private Message Find Members Posts Add to Buddy List Admiral Joined: 27 November 2017 Location: Sag Harbor, USA Status: Offline Points: 1112	Post Options Post Reply Quote Arcadia Report Post Thanks(0) Quote Reply Posted: 04 July 2025 at 14:50
	Also, be aware that even though the starter battery will deliver many hundreds of amps during starting, the energy consumed is only about 1 amp hour so any size charger will do the job quickly. The bigger issue is that the Volvo D3 on our boats is computer controlled which consumes power as does the blower fan. So a charge current of 25 amps to the engine bank would be sufficient.
	Leon / ARCADIA 2018 Hanse 588 Sag Harbor, NY

Arcadia Members Profile Send Private Message Find Members Posts Add to Buddy List Admiral Joined: 27 November 2017 Location: Sag Harbor, USA Status: Offline Points: 1112	Post Options Post Reply Quote Arcadia Report Post Thanks(0) Quote Reply Posted: 04 July 2025 at 14:54
	Yes Marsella. That would work and eliminate the combiner. But as I mentioned, it’s important that the alternator charge cable be moved to the services bank. Otherwise the service bank would not get charged with the engine running.
	Leon / ARCADIA 2018 Hanse 588 Sag Harbor, NY

Arcadia Members Profile Send Private Message Find Members Posts Add to Buddy List Admiral Joined: 27 November 2017 Location: Sag Harbor, USA Status: Offline Points: 1112	Post Options Post Reply Quote Arcadia Report Post Thanks(0) Quote Reply Posted: 04 July 2025 at 14:58
	Also, for anyone going in this direction, moving the blower supply voltage to the service bank will greatly reduce the running current supplied by the engine battery (about 20 amps). And because the service bank has the high current alternator connected, the extra load would be minimal there.
	Leon / ARCADIA 2018 Hanse 588 Sag Harbor, NY

Arcadia Members Profile Send Private Message Find Members Posts Add to Buddy List Admiral Joined: 27 November 2017 Location: Sag Harbor, USA Status: Offline Points: 1112	Post Options Post Reply Quote Arcadia Report Post Thanks(0) Quote Reply Posted: 04 July 2025 at 15:01
	All this being said, there are many ways to configure the charge sources and the various battery banks and how they interact. No one method is correct, but the important thing is that it works well and addresses the specific needs and concerns of the owner.
	Leon / ARCADIA 2018 Hanse 588 Sag Harbor, NY



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