LEAD CARBON BATTERY FAILURE, LESSONS LEARNED

21 February 2026

The three 12 volt 120 amp Lead Carbon batteries installed on Truce worked flawlessly for 22 months, I was happy.   Then, out of the blue, over a period of just a few days all three batteries failed.

Fortunately, at the time of failure I was in a protected anchorage in the Philippines and could manage the problem locally.   If the failure had occurred on passage it would have been more stressful.   I would have been reduced to operating Truce on the 75Ah radio battery and engine start battery.     While inconvenient, the show would go on, after all, back in the good old days I happily sailed with a 100 amp house battery and warm beer.   Other cruisers with a less robust back up could potentially have faired much worse, with failure of navigation systems, water makers, cookers, communications etc.  

As the batteries were warranted for 24 months I put the battery supplier on notice.   Then I set about procuring some replacement batteries.   Fortunately, the Philippines is building a lot of domestic solar and I was able to procure three 12 volt 100 Amp gel solar storage batteries (an unbudgeted cost). Within a few days normal power on board had been restored. The new battery capacity is less than the lead carbon batteries, I have to be careful with the power use, including switching the fridge off for a few hours at night occasionally.   Thankfully, as my fridge is well insulated (and well stocked) I was still able to keep the beer cold.

My choice to replace my flooded cell batteries and switch to Lead Carbon batteries was based on faster recharge, lower depth of discharge, less maintenance, and more cycles before degradation.   I was led to believe that lead carbon batteries were drop in replacements for flooded cell batteries (wrong).   This seemed a robust, long lasting alternative to flooded batteries without going to Lithium type batteries.   I was happy to think that LC batteries would be the last batteries I would buy for the boat in my lifetime.   The marketing blurb and information from marine electricians was very convincing on this matter.

As a single-handed sailor, I am intimately familiar with all the systems on board, most of which I have installed myself.   Simplicity, reliability, quality, robustness with ease of maintenance is the route I always try and take.   When I first noticed the decline of battery power I was incredulous, how could this be?   I immediately assumed that something was wrong with the electrical system.   After a thorough check around everything seemed to be normal.   Solar controllers were working correctly, the DC/DC converter was working normally, battery temperature sensor and compensation was OK, no phantom loads, the electrical system was all good.   I isolated the batteries one by one and they all showed significant loss of performance and inability to hold a charge under any load.   I was not happy.

On Truce the electrical loads are moderate.   In the tropics the Isotherm fridge cycles about 7 or 8 times per hour at 7A.   I also have a rice cooker, 19A for 35 minutes and a 12V water maker 17 amp (only used when good solar available or with the engine running).   The big load items are not usually run concurrently but fridge, rice cooker and fresh water pump coming on at the same time could result in a short period of higher current draw which I don’t think would ever exceed 40A for a brief period of time.   There are also 2 small inverters on board, one is 110V – 250A and the other 220V – 300A, used for computer, phone, electronics charging.   In practice the batteries don’t usually see more than 25% discharge.   They are certainly not over stressed or deeply discharged.  

Truce has 350 watts of solar panels feeding through three smart Victron Solar controllers.   There is also Victron smart battery temperature monitor that feeds battery voltage and temperature information to the solar controllers.   The Victron smart DC/DC (30 Amp) controller charges from the alternator to the lead carbon battery bank.   The charge current to the house batteries does not exceed 30 Amps.

Why did the lead carbon batteries fail?   After many email exchanges with the battery supplier they seemed to indicate that it was all my fault.   Why is it that whenever something goes wrong its always my fault!   It was agreed that I would bring one battery back to New Zealand for inspection by the supplier to ascertain the cause of the failure.   I spent some time securing the homeward bound battery with timber chocks and dyneema lashings. Lead carbon batteries are very heavy, it would not be nice to have it moving around in bad weather.

Eventually I returned to New Zealand, the supplier opened up the failed battery for inspection.   The suppliers findings are detailed below exactly as they were passed to me: –

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Inspection findings on failed battery

One of the failed 120 Ah lead carbon AGM batteries was cut open for inspection. The positive plates showed very dry, brittle active material rather than a dense cohesive surface.
The grid pattern was clearly visible through the paste, indicating shrinkage and loss of contact between paste and grid. There was extensive cracking of the paste and visible areas where material had broken away, especially near plate edges.

There were no obvious signs of internal plate shorting, dropped plates, or external mechanical damage.
This condition is typical of long term operation at elevated temperature with relatively high charge voltage and extended absorption or float periods for VRLA batteries.

Operating environment

The cruising grounds have ambient and seawater temperatures commonly around 30 to 35 °C.
In a confined compartment with restricted airflow the internal battery temperature will be above ambient during charging and for some time afterwards.
The daily load profile is modest, with overnight consumption usually less than 25 percent depth of discharge. Solar contributes most of the charging, with the DC DC charger adding charge when motoring.
The boat often sits on a mooring unattended with only the fridge, pumps and lights as loads, while solar maintains the bank close to full charge for long periods.

Assessment of failure mode

Lead carbon AGM batteries are more tolerant of partial state of charge than standard AGM, but they still behave like other VRLA products with respect to temperature and over charge.
VRLA battery life typically halves for roughly every 8 to 10 °C above the design reference of 20 to 25 °C.

In this case the combination of factors is important.

1. Elevated temperature in the battery compartment for long periods due to tropical ambient conditions and heat build up in an enclosed space.
2. Charging profiles that hold the bank at absorption and float voltage while the batteries are already hot.
3. Long periods at or near full state of charge, driven by the solar system while loads are low.
4. Restricted airflow around the battery cases and no air gap between batteries, which traps heat and raises internal temperature further.

Under these conditions, charge energy that cannot be stored efficiently is converted to heat in the positive plates. The plates expand and contract with temperature changes. Over time this dries the paste, causes cracking and loss of adhesion to the grid, and leads to shedding of active material.
The plate condition observed is consistent with this thermal and over charge stress, not with simple sulphation from under charging or an isolated manufacturing defect.

Once one battery in a parallel bank loses capacity, the remaining units carry more of the discharge and charge current. Their internal temperatures rise further and the same failure process accelerates. This matches the owner’s experience of one unit failing first, followed by rapid deterioration of the remaining pair.

Ventilation, air gap and thermal management requirements

From a battery perspective the primary driver of the failure is the combination of high operating temperature and limited ability to remove heat from the battery compartment. Both ventilation of the space and spacing between individual batteries are important.

Industry manuals for VRLA batteries typically specify that batteries should not be installed hard against each other. A free air space of about 5 to 10 mm between adjacent units is recommended to allow heat to dissipate. Some guides for deep cycle blocks recommend slightly larger gaps, but the common minimum is in the 5 to 10 mm range.

In the owner’s photos:

1. The batteries appear to be touching or nearly touching, with no clear air gap.
2. Polystyrene packers are used between cases, which act as thermal insulation.
3. The top of the compartment is partly covered by a cushion, which further restricts the escape of warm air.

This installation removes the intended cooling path between batteries and traps heat around the cases. In tropical operation that will significantly raise battery temperature during any charging period.

These spacing and ventilation requirements are not specific to lead carbon. They apply to all VRLA products, including standard AGM and gel designs. Gel cells may gas less under normal charge, but they are still VRLA lead acid batteries and high case temperature will shorten life in the same way. sonnenschein.org+1

For similar AGM or other VRLA installations in hot environments the key requirements from a battery point of view are:

1. Provide a clear air gap of at least about 5 to 10 mm between adjacent batteries so that air can circulate and heat can escape from the sides of each case.
2. Ensure vents and openings in the compartment remain unobstructed in normal use. Items such as cushions or stowage should not block warm air from rising out of the space.
3. Where ambient temperatures are regularly above 30 °C, provision for airflow, for example a small fan linked to charging, is strongly recommended so that battery case temperature remains as close as possible to cabin temperature.
4. Avoid adding insulating materials such as solid foam packers between battery cases unless there is still a clear air gap around each unit.

Any physical changes to ventilation hardware, battery mounts, cabling or protection devices must be carried out by suitably qualified marine electrical or boatbuilding staff. Our advice is limited to the effect of temperature, airflow and spacing on AGM and other VRLA battery life, and to the general need for improved circulation around the bank.

Conclusion

The inspection of the failed 120 Ah lead carbon AGM battery, together with the reported operating history and installation photos, indicates that the primary cause of failure is long term operation at elevated temperature combined with relatively aggressive charging, long periods at high state of charge, and inadequate ventilation and spacing of the battery bank.

The cracking, shrinkage and shedding of active material on the positive plates are consistent with thermal and over charge stress on VRLA batteries in a hot, confined environment. The lack of any meaningful air gap between batteries and the use of polystyrene packers between cases would have increased battery temperature further.

For similar tropical cruising applications using AGM or other VRLA batteries, acceptable service life depends strongly on actual battery temperature, time spent at absorption and float voltages, and whether the bank is installed with adequate ventilation and the recommended air gaps between individual batteries.

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The report above is correct on most points.   The lead carbon batteries were positioned in individual battery boxes that previously housed the previous lead acid batteries.   The batteries were not touching but did have polystyrene wedge packers to prevent movement in the individual boxes.   The battery compartment is open to the bilge where the air is cooler, but with high sea water temperatures in the tropics the battery compartment will never be cooler than the seawater, around 30 degrees C.   There is no forced ventilation in the battery locker and as the report states heat will build up.

When I received the batteries I did not receive any instructions, which is very common when buying batteries.   Although I was not concerned at the time, with hindsight I should have been more vigilant and not assumed that the batteries would be a drop in replacement for lead acid batteries.   My previous experience with flooded batteries in the tropics was that the electrolyte just needed topping up more frequently.   So, my takeaways from this experience are: –

1. Always get operating and installation instructions from the supplier and follow them
2. Make sure battery compartment is sufficiently ventilated and monitor temperatures
3. Do not leave batteries on float charge for long periods

The battery supplier, HBC New Zealand, did honor the warranty and offered to provide three replacement batteries.   Thank you to HBC, a good conscientious company.     However, after my experience with Lead Carbon batteries I have decided not to replace like for like.   I am now switching to Lifepo4 batteries.   So far the installation and operation of the Lifepo4 batteries has been as expected,   but its still early days.

I hope the above is helpful to other cruisers and sailors. Since my battery failure I have been contacted by others who have also had Lead Carbon battery failures. Some had their batteries replaced under warranty but one poor unfortunate had a failure when the batteries were just out of warranty. I don’t know the details of the other failures so cant comment if it was user fault or battery fault. Its just pays to be careful and do your homework when installing a new battery system.

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