The Offshore Voyaging Reference Site

Solar: Lead-Acid Battery Saver, Potential Lithium Wrecker

The above is a graph of battery voltage on our J/109, currently still out of the water but uncovered.

As you can see, each day shortly after dawn our solar panel starts charging and since, other than the first day when I was aboard and using power, there are no significant loads and the batteries are fully charged, the voltage rises rapidly to ~14.4 (temperature dependant): acceptance for our AGM lead-acid batteries.

And, after a predetermined time, and because the batteries were full anyway, our solar regulator drops the charge voltage a bit, and then at dusk it shuts down.

Lead Loves It

This is absolutely wonderful for lead-acid batteries because that chemistry is:

  • Tolerant of being held for long periods at acceptance voltage, even when already full.
    • Lead-acid battery internal resistance rises a lot with state of charge, so even though the voltage is still at acceptance very little current flows (amps) so little or no damage is done.
  • Quickly wrecked due to sulphating from being left in a partial state of charge.

We might even say that solar makes lead-acid batteries wonderful again.

Lithium Not So Much

But this same pattern would shorten the life of lithium batteries because:

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Dan Tisoskey

During a refit this year, I removed three lead acid and replaced with two, 100amp lithium. Starting battery is still lead acid and the alternator charges the starting battery and the house bank is charged by one 30 amp Victron DC to DC charger and 150 watt solar array with Victron charge controller.

I am wondering if I sent the solar charge the the starting lead acid and used the DC to DC charger to charge the house bank and use the Cerbo GX to set the DC to DC charger to regulate the solar?

Just a thought…


Dan Tisoskey

I installed Li Time (low cost Amazon) I used these in the past with great results. I do not live aboard and my longest cruises are around three nights at sea. I have a backup navigation (Ipad) to my 9″ Flir chartplotter, Octopus ram, LED lights. I would be able to jump critical loads to the starting bat if needed. My issue is I have so little draw on the batteries I think 200 AH’s is too much.

I am thinking about installing Technautics CoolBlue fridge which would burn around 25 amps per day. Still not bad considering my solar panels should give that back each day (and on cloudy days this may help my lithium bank live longer as I am not sending a charge to fully charged batts.

Henrik Lundin

I have always assumed that this function (sensing charge current at a specific voltage and terminating charge) is available if you combine a Victron smart shunt with a Victron dc-dc charger.

In that case it might be a more frugal investment for a small lithium system in comparison to the Victron Cerbo.

Henrik Lundin

I spent some quality time this morning with good coffe and some victron manuals.

Without any satire I can say that I love good manuals. This is maybe… 70% of the reason why I tend to buy victron products.

Below I am paraphrasing from the manual for Orion XS:

With VE.Smart Networking current sensing can be added to your Orion XS when paired with a battery sensor like a BMW or Smartshunt.

The sensed battery current is used for the tail current setting.

My conclusion is that a victron smartshunt/bmw can work together with the victron DC-DC chargers. Although I am not sure if this is applicable for older models.

ps. I also think that there is a method to set up a signal wire from the relay on the BMW 712. But I would like to try it out in real life before I am a 100% sure.


BMW 712

relevant chapters:

4.4.3 Charger Mode – Battery settings

4.6 Remote Sensing with VE. Smart Networking

Stein Varjord

Hi Henrik and John,
The SmartShunt, and indeed the BMV 712 as well, can cooperate with Victron chargers that have VE net or similar. They can both use Bluetooth as well as a VE cable connection. The BMV 702 is the same unit as the 712, but has no Bluetooth so it can only use the cable.

The info from the battery monitors will by default become the preferred battery sensing method, ditching the DC-DC, AC charger or solar regulator sensors. The units with such functionality can also use the info to adapt the charging.

However, to remedy the issues mentioned in this article, you need to go into the expert settings, which isn’t always for the faint of heart. I’m also not entirely sure that the currently available settings options are sufficient. I haven’t researched enough to become certain, but I think there are still issues.

At the moment, I’ve set the saturation time (constant Voltage, 13,8V @ diminishing Amps) to 10 minutes, which is plenty for batteries that are properly balanced. Still this isn’t a full solution to the problem. I’m still looking for a better method. I don’t have a Victron BMS, so it’s not a walk in the park.

Matt Marsh

There really is no substitute for having the BMS – the only automation system aboard that really knows what’s happening inside the battery – control the shutdown of charging sources.

Most electric cars now have a user-intent-aware BMS. If you aren’t planning a long trip that day, the BMS has the option to terminate charging at about 80% full. And if the car is going to be mothballed for six weeks, holding the charge closer to 50% might be preferred. Only if you tell it that you need the full capacity for the next morning’s run will it bring the state of charge to 100% overnight. For maximum longevity of a boat’s LFP house bank when solar panels or shorepower chargers are in use, a similar capability may be desirable.

Eric Klem

Hi John,

Totally agree on solar being great for lead acid and potentially very detrimental to lithium if not managed correctly.

I am curious what your charge profile looks like. It appears you may be going to bulk briefly then holding at absorption for a while, is that right? If so, why is it holding in absorption for so long? Or am I misinterpreting and you have your float set that high?

I have been kind of debating how to program ours. The last 2 years we have used a Victron SmartSolar controller. When just weekending, I use their adaptive absorption which seems to do a decent job. We transition from bulk to absorption at a pretty high state of charge when the alternator isn’t helping but when the alternator is on, that kind of confuses it and we end up floating early. Even if we don’t get fully charged that day, the next day we certainly will so I don’t worry about it. Interestingly, our charger seems to put 4-6Ah into the batteries everyday when the batteries have no load (there is a DC to DC charger that is always on but draws less than 1/4 that). However, when cruising we switch to a manual absorption setting based on time as the adaptive one floats early more than I like and I would rather over than undercharge our lead acids.


Eric Klem

Hi John,

Thanks, that makes sense. It is good to hear that you are comfortable overcharging AGM’s that much, I would definitely be comfortable overcharging flooded batteries that much, we would just add a bit of extra water. Most of my experience with AGM was in applications where we were struggling to generate enough power so undercharging was the concern and not overcharging.

We used a Genasun charger for our panel for 9 years on Rod Collin’s recommendation and there didn’t seem to be any ill effect on the batteries, at the time we got it there were not as many options and it seemed to be the best for the batteries. We switched to the Victron when the Genasun stopped working and I opened it up to find that one of the through hole solder points had the solder melted out. Our panel was the max rated size for that controller so rather than soldering it back together, I decided to swap as there were other signs of things getting warm.