davidq avatar image
davidq asked

HELP - Leaking starter batteries / rotten egg smell - Travelling across Africa - Stuck

Hello Community,

Hope someone can guide me :-)

We are in a professionally built expedition truck with Victron systems. Over the last two days we have been smelling rotten eggs. I started with the toilet but ended up at the starter batteries :-(

Both starter batteries are leaking (2 x 12v series).

Q1 Is it usual for both starter batteries to fail together or is this symptomatic of an external issue ( overcharging issue or something else)?

We are parked up (Corona 19) with a shore supply feeding a Pheonix Multiplus that charges our leisure batteries and starter via a Cyrix-i 12/24-120 bidirectional voltage sensitive relay. A Victron MPPT solar charger is a second charging source (added a month ago to replace a PWM unit).

About two days ago the inverter displayed a Bulk Protection alarm. And I reset it. I didn't know what this was, but the community has told me that it means the max bulk charging time of 10 hours was exceeded. I suppose this is associated with my problem.

I noticed yesterday that my MPPT was generating 17A, but the BMV indicated only a few amps charging the onboard leisure batteries and I wondered where the rest was going as I had not much load switched on. I guess it was going into the started batteries.

Q2 Can anyone through some light on what is going on? Have the batteries failed as a result of a system issue or of their own accord and just need replacing?

Many thanks in advance to responders.



Multiplus & MPPT have identical charge settings (bar current limit):

Vab = 28.8v

Vfl = 27.6v

Current limits are 45A and 30A respectively.

Voltage across starter battery terminals (chargers off): 24.8v

Voltage across leisure batteries (from BMV) (chargers off): 25.5v

Voltage across starter battery terminals (MP charger on): 27.6v and climbing slowly

Voltage across leisure batteries (from BMV) (MP chargers on): 27.7 and climbing slowly


The starter batteries are fully sealed maintenance free units.

The leisure batteries are Victron AGM 220AH (2 in series for 24v)


Starter batteries last replaced in May 2016.


System Diagram

1 = Starter Battries

6 = Cyrix-i 12/24-120

9 = BMV 602s shunt

8 = Leisure batteries

11 = Inverter 24/3000/70 Phoenix MultiPlus


I measured the voltage across each of my series 12v starter batteries: 13.35v and 11.25v.

The latter seems low given they have been on charge for 24hrs +.

Does this mean that the starter battery voltage is low overall 24.6 and this would mean battery voltage does not reach Vab and stays in bulk or just make absorbtion and draws lots of current. This causing both batteries to overcharge and leak?

Have I answered my own question? I don't want to rush out and buy two new starter batteries until I understand the problem. They cost $$$.



As suggested by respondents, I charged the under-voltage battery overnight on a 12V charger. This morning the on charge voltage was 12.1v and this fell back to 11.4v within 20 mins after being taken off charge.

The other battery was at 13.06V this morning.

Q1 I guess this means this battery is toast and needs to be replaced?

Q2 I intend to replace the pair (they are 4 years old) - any reason not to?


I replaced my starter batteries and everything is fine. I have ordered two battery balancers, one for my starters and one for my house batteries.

Thank you all respondents for helping me out.


battery chargingstarter battery
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marekp avatar image marekp commented ·

At comment 6:

Yes one is toast, and yes you have to replace both since they work in series.

Keep the other one as backup, but keep it always charged.

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Warwick Bruce Chapman avatar image Warwick Bruce Chapman marekp commented ·

And when you replace them, please add a Victron battery balancer to each series bank to keep the two batteries balanced in the long run.

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davidq avatar image davidq Warwick Bruce Chapman commented ·

Hi Warwick, these are my starter batteries that come with the truck. I suppose the manufacturer did not judge a battery balancer necessary (for charging from alternator), but now I have my onboard Victron charging them when stationary.

Can you install the balancer on a vehicle starter battery system?

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4 Answers
michelg avatar image
michelg answered ·

Hi David ! Check the electrolyte of the batteries, if it is crystalline like water or muddy. Also, distilled water might be missing in one or both batteries, check it too. When charging the batteries, verify if they are getting hot. A cell might be cracked or damaged in one of them.

Could you report models and setup ? Also, when charging check the voltage arriving to the batteries.


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davidq avatar image davidq commented ·

Hi Michel, thank you for rsponding. Iv'e updated my original post with charge profiles and battery types. Starter batteries are sealed - so no means of inspection.

The voltage across the starter batteries is 27.6v on charge from MP.

Would both fizzle/vent if just one had a cell issue?

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marekp avatar image marekp davidq commented ·

"Would both fizzle/vent if just one had a cell issue?"

Yes. If one cell goes bad (shorted = 0V) you have now 11 cells instead of 12 in series.

Those 11 cell are being charged as 12, therefore all of them are overcharged.

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davidq avatar image davidq marekp commented ·

Thanks Marek - that makes sense.

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spirou avatar image
spirou answered ·

It's also possible one developed an internal short, leading to overheating, gassing etc. Seen a post recently on another forum of a battery explosion under the seat with the other one seemingly fine. Apparently the bad one was nearly dry by the time the top blew off.

A few years ago I also had a gel battery with a short, audibly boiling, bulged sides, some acid release through vents etc. Unless something is wrong you probably wouldn't smell any acid from a sealed battery.

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davidq avatar image davidq commented ·

Thank you Spirou. I'm charging the battery now.

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Matthias Lange - DE avatar image
Matthias Lange - DE answered ·

The batteries are out of balance, try to charge them individually with a 12V charger for around 12h.

Disconnect the charger after the charging is done, wait a hour and measure the voltage. If the batteries are ok the voltage should be around 12.7V.

If the batteries are ok you can connect them together and maybe install a Battery Balancer.

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davidq avatar image davidq commented ·

Thanks Matthis. I've put it on charge.

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mondotv avatar image
mondotv answered ·

So you have two 12V batts in series but essentially you are treating them as one big 24V battery. Here’s the issue. If one battery goes bad the charger has no way of knowing because there’s no BMS monitoring individual cells to shut off the charger. So you have the situation you have where one battery is getting seriously overcharged while the other one never reaches full charge (most likely because it has one bad cell - a 12V sealed lead acid has six individual 2V cells in series). We don’t normally bother balancing or individually monitoring SLA or FLA because it’s already 6 unmonitored cells in series. I would take both batts off charge immediately, remove the undervolted battery and see if you can bring it back up to charge on a 12V charger - I suspect not as the battery is probably internally shorted and that is where the current is going in the form of excess heat. You’ll probably need to replace that battery and keep an eye on the other one to ensure it hasn’t been damaged as well.

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davidq avatar image davidq commented ·

Thank you MondoTV,

Yes, I took them off charge yesterday and placed a 12V charger across the one with the lower terminal voltage overnight. Its currently sitting (with the 12V charger on) at 12.1V. As Matthias suggested below, I will soon disconnect, wait an hour and measure the terminal voltage. I'll report back soon.

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marekp avatar image marekp davidq commented ·

Something is wrong with this battery.

Probably one cell is shorted.

After being charged overnight and still at 12.1V. :(

Fully charged cell and charging cell gets to over 2.4V, so 5 x 2.4=12V.

Your is now at 12.1 so you have 5 fully charged cells and one shorted.

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