Batteries Overview & Selection

There are 3 main types of batteries available, choosing which one involves digging out relevant information.

The HR 41 came to us with Gel Cell batteries, the Maxi 95 came to us with Wet Acid batteries. Both worked well, the costs were different but so were the life spans.

We never intended to get into the “battery selection” game. With the Maxi 95 we replaced the Wet Acid batteries that it had come with, we also put in an advanced charging system for them. The HR 41 came with Gel Cells and an advanced charging system. We didn’t really do any research on batteries and battery types until we went to replace the Gel Cells on the HR 41. We decided to replace the Gel Cell batteries when we made a major overhaul of the boat as although the batteries were not showing any sign of failing, we were unsure of their age and wanted to replace them before we were forced to.

There seem to be a variety of outspoken opinions on battery types, sifting through them is a bit tedious as they tend to get into ranting. One useful site we found was the Von Wentzel website – he didn’t reach the same conclusion we did with respect to battery type but he was evaluating for a different user profile. We did use his approach for evaluating and selecting a battery type.

The two “new” battery technologies that have become available are AGM and Gel Cell. Simply put, the advantage for AGM is they can be re-charged faster and be depleted further than Gel Cells while suffering less lasting effects. For the Gel Cells the advantage is that more discharge cycles are provided which means a longer lifetime for the battery. AGM batteries are very good for applications like emergency vehicles where speedy re-charging can be critical and lifespan is not as important as the AGM batteries can be replaced back at the garage. For a cruising boat which doesn’t want to replace the batteries any sooner than necessary, the longer lifetime is a factor. Also with constant re-charging from wind and solar we were never planning on discharging the batteries to low levels. And faster recharging was not a factor that weighed heavily in our decision, after all one aspect of cruising is slowing down your life, not speeding it up.

An easy way to think of the 3 battery types is:

Battery Type Pros & Cons
Wet Acid least expensive, require maintenance
AGM faster charging, deeper discharging, shorter life than Gel Cells, maintenance free
Gel Cells longest life of the 3 types, maintenance free

Using the above table as a summary we needed to decide which battery types to select for our house batteries.

We needed to find some numbers in order to make a numerical comparison (something that seems to be missing in most online discussions). One manufacturer’s data sheet listed the lifetime of their battery types as:

Battery Type Number
of Cycles
Increase
in Lifespan
Notes
Wet Acid 950 The life cycles were measured using a 30% discharge level (i.e. battery level varying between 70% and 100%).
AGM 1,400 47% more than Wet Acid batteries
Gel Cell 2,450 158% more than Wet Acid batteries
75% more than AGM batteries

The numbers tend indicate that you get the best “bang for your buck” with Gel Cell batteries but they don’t cover everything because there is no cost listed. The same table except comparing relative cost is:

Battery Type Relative
Cost
Increase
in Cost
Notes
Wet Acid 1.0 To make up this comparison
we selected four 100 amp-hour batteries from the same vendor and calculated the relative prices per amp-hour. We used a quality marine battery for the Wet Acid candidate (i.e. not the budget cheapest battery available),
and used 2 AGM batteries – the most expensive and the lowest priced and then averaged the two prices.
AGM 2.35 135% more than Wet Acid batteries
Gel cell 2.4 140% more than Wet Acid batteries,
5% more than AGM batteries

This table seems to suggest that either AGM or Gel Cells are over twice as expensive as a reasonable quality (i.e. not budget) Wet Acid battery. But to really get the right parameter, we need to combine the two previous tables into:

Battery Type Relative
Cost per Cycle
Relative
Cost for Life of Battery
Wet Acid 1.0 1.0
AGM 1.59 59% more than Wet Acid batteries
Gel cell 0.93 7% less than Wet Acid batteries
66% less than AGM batteries

So it appears that using our evaluation criteria the Gel Cell batteries are most appropriate (and cheaper in the long run).

Anyone who has read the excellent  Von Wentzel website will notice that we reached a different costing and conclusion than he did (he selected AGM batteries). This can be attributed to:

  • our 50 amp alternator is minimal (most boats, including Von Wentzel, run 100 amp alternators or bigger)
  • reduced battery charging time is not our priority (AGM batteries excel in accepting higher charge rates)
  • we have over 100 amp-hours charging from wind and solar per day (based on only an 8 hour period per day)
  • due to our wind & solar charging we rarely discharge our batteries below 10%

For us increased battery lifespan was more important than speedy charging with high charging rates. It’s not that either is wrong, it’s more a matter of priorities. With our wind and solar inputs we do not need to either install a larger alternator or maximize our charging rates.

Observation

Maringret actually has 3 battery banks on board, due to various factors they are not necessarily the optimal type:

Voltage Optimal Type Actual Type
House battery 12 Gel Cells Gel Cells
Engine Starting battery 12 AGM Gel Cells
Bow Thruster battery 24 AGM AGM

The three usages are quite different and it makes sense to have possibly had different battery types for each one. Both the engine starting and bow thruster batteries are applications involving delivering high current and accepting a high recharge current so as to be ready for the next use as quickly as possible. The house batteries (combined with the solar and wind recharging) are not tasked with high current provision but rather low current over as many years as is possible.

The complicating factors are that the Bow Thruster battery bank runs at 24 volts and is totally separate – it has dedicated chargers for both the alternator and shore power. Meanwhile the engine and house batteries share chargers for both the alternator and shore power. Switching the engine battery to AGM would have made sense but added yet another pair of charging units. Charging units will charge various different types of batteries but only one type at a time.

  • The main lesson we learned was one of costings, for our priorities the Gel Cell batteries were most appropriate. Due to the much longer lifespan of the Gel Cell batteries, our costs are actually lower in the long run than they would be with either of the other battery types.
  • Our priorities are based on sailing in sunny, windy locations with short nights (i.e. short periods without sun). Were we were to sail in winter environments (i.e. reduced hours of sun) then possibly AGM batteries would be a better fit for us.

  • We replaced all batteries in 2011.
  • By the summer of 2013 the Victron engine gel battery was no longer able to keep a charge and we replaced it  All the other batteries are operating optimally after the first year.
  • As of 2019 the AGM batteries for the bow thruster have packed it in and need replacing. They last about 9 years. The Gel Cells for the house battery bank are still performing as they did when we first installed them.


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