Maringret’s batteries were 10 years or older and starting to show a decline in ability to retain their charge.

Maringret came with four 120 Ah Varta batteries. Rather than normal wet acid batteries these were gel cells. For the first few years of our ownership they had functioned flawlessly. We were not sure of their age but they were starting to show inability to retain charge. We had no prior experience with gel cells.

House and Engine Batteries

When we purchased Maringret we knew that eventually we wanted to increase the amount of battery storage. One lesson we had learned with the Maxi was that one can never have too many batteries. Battery capacity is analogous to tankage in that when you are capturing energy from wind and sun you can only store up to the capacity of the batteries – after that everything is lost.

Of the four batteries of 120 Ah one was dedicated to engine cranking leaving the other three to cover the house requirements. 360 AH may sound like a lot but compared to the Maxi where we had 340 Ah for the house (140 Ah for fridge and 200 Ah for the house) it was not much of an upgrade. Also the HR 41 had radar, a bow thruster, and a anchor windlass to run – none of which were on the Maxi.

When we bought the HR 41 and found she had gel cell batteries we were not sure what to think. Having never dealt with the technology we were unaware of it strengths and weaknesses.

As battery technology is a fairly obscure and changing topic we never found any general distribution publishing on the topic aside from manufacturer or sales brochures. We checked various online forums (e.g. Cruisers Forum)and found information of varying trustworthiness. It seems that as there are no editorial or review procedures on the forum groups and that you have to look for repetition over time. Of course this would also include posting by zealots which we did see evidenced – how do some of those people make over 10,000 postings on each of 2 or 3 forums? Do they actually use their boats or just post about having a boat? In addition to the forums we collected PDF files from manufacturer websites, in some ways it is easier to use this information as you at least know where their bias will lie. Something we noted from the forums were that some of the major manufacturers were reported by cruisers as not honoring their warranties, or insisting that the batteries be returned to the point of sale or to the manufacturer rather than the local dealer. Where such reports seemed common we decided to strike those manufacturers from our list.

The impression we formed based on our research was the following:

  • newer technology batteries offer benefits over wet acid batteries (e.g. maintenance free) but do cost more
  • any newer battery technology must have its own charging system or it will never deliver it’s benefits
  • there is a certain amount of dissent on these topics

We found out that once we phoned and spoke to vendors or manufacturers that their staff spoke mostly in terms of “suggestions” and “recommendations”, it seemed that no one was coming out and saying that one technology was definitive as the preferred solution for a given problem area. In fact one of the dealers for the batteries we finally went with suggested that if we had had good results with the gel cells on our boat, that we stay with them.

One source from the late 90s is the Von Wentzel website (click here) who evaluated the battery technology in the late 90s and evaluated them against his priorities. He has a good list of links to further battery sites. Another good article is the magazine article “The ABCs of AGMs” by “Steve C. D’Antonio” printed in “Ocean Navigator” magazine, issue No. 142, published November/December 2004. We were able to find a scanned version of the article on a website although that was later taken down. D’Antonio has his own website located at (click here) although he did not have this article as a download last time we looked. The article is quite good for what it is, it does not really cover much about the choice between gel cells but rather as the title states is about AGM battery technology. Also it is now more than 5 years old and it would be nice to see it updated and broadened to compare the main three battery technologies. The largest number of battery websites we found were for off-grid applications in buildings or cabins. A lot is the same but weight and volume requirements are more critical on a boat as well as the focus is not as much on providing AC current for everything.

In our search we had decided that we did not wanted to have to maintain batteries – this left us with AGM or Gel cell battery technologies. Our existing charging systems were able of charging any type of battery so switching from gel to AGM was not a cost issue for us and purchase prices for gel or AGM seemed quite similar. We found a corporate publication from Victron Batteries called “Batteries Energy Unlimited” which was not on the corporate website but which their dealers provided as a download file. Tracking documents on the internet as websites are reorganized is an endless task so we also list the document control number which appears to be “SAL064108020 / 06-08”. Perhaps Google will be able to find the document based on the control number.

One thing we liked about this document was that they compared technologies in their own batteries, below is a excerpt from that publication:

—————— Cycle Design Life —————— ie41_victronBatteryCycles
Product Name Float Design Life 100%
12 Volt Deep Cycle AGM 7-10 years 200 cycles 400 cycles 900 cycles flat plate AGM
12 Volt Deep Cycle GEL 12 years 300 cycles 600 cycles 1,300 cycles flat plate GEL
2 Volt Deep Cycle GEL 20 years 700 cycles 1,200 cycles 2,400 cycles tubular plate GEL

We felt we could trust this document as a technology comparison as Victron was testing its own products and had no advantage we could think of in discounting one of its own products. This table does not include two pieces of data that would be interesting: firstly comparative figures for wet acid batteries and secondly the price per amp-hour of battery capacity for each of the technologies. We were able to extend the table using retail prices as of late 2010 as follows:

Product Name Relative Cost per Amp-Hour
12 Volt Deep Cycle AGM 0.90 about 10% less than Gel Cell batteries
12 Volt Deep Cycle GEL 1.00
2 Volt Deep Cycle GEL (approx) 2.00 roughly twice the cost of 12 volt cells
12 volt Deep Cycle Wet Acid 0.33 as low as 1/3rd of the price for Gel Cells
  • There is a premium of 10% for Gel Cell batteries over AGM batteries
  • It was a bit more complex to generate a costing for 2 volt Gel Cells: the smallest 2 volt battery made is 800 Ah! By coincidence that current capacity is approximately the same as our total battery bank but at only 2 volts. So if we put 6 of the 2 volt batteries batteries in series we would end up with 800 Ah at 12 volts – roughly the same as our complete battery bank. Based on this the costing using 2 volt Gel Cells would be approximately twice that for 12 volt Gel Cells.
  • For the Wet Acid battery price we used an average quality battery intended for automotive use, much cheaper batteries were available. If a premium brand “marine” battery was used then the relative cost can be as high as 75% of Gel Cells or more. There is a much greater range in prices for wet acid batteries than for other battery technologies.

When we found the Victron document we were very interested that the Gel Cell batteries are listed as 50% more cycles than AGM. One advantage that AGM batteries have compared to Gel Cells is they can take on charging current much faster which means they can be recharged in much less time. For us where our batteries are rarely below 25% discharge and wind and solar are almost always providing charging a faster recharge time is less important than a greater number of discharging cycles combined with a design life of up to 50% more. From the table above it is obvious that even better performance is offered by the 2 volt gel cell batteries – approximately twice the performance but at twice the cost. One advantage of the 2 volt Gel Cells over the 12 volt variants would be that the time until replacement would be twice as long, very important for unattended operation sites but not so critical for us. Once we costed out the 2 volt Gel Cells we realized that they offered twice the lifetime for twice the cost. Their casing shapes are quite different from other batteries and we would have required modifications to fit them in addition to the cost of the extra wiring costs connecting the separate batteries.

In the end we decided on Victron 12 volt Gel Cell batteries, partly due their high reputation and partly due to a very competent dealer. We decided on Gel Cell batteries due to the combination of increased design and cycle lives along with absence of maintenance. The order for Maringret was:

120 Ah 150 Ah Total Comments
House Batteries 5 750 Ah We doubled the house battery capacity
Engine Battery 1 120 Ah The same cranking battery capacity

Bow Thruster Batteries

The workload for a battery is much the same whether for engine starting or bow thruster operation. Maringret came with four wet acid 12 volt 800 CCA starting batteries wired up as two 24 volt banks for the bow thruster. As with the house batteries we did not know the age of the batteries, while the gel cells were slowing declining the wet acid cells suddenly fell to 2.4 volts. This may have been due to a cell failure within one of the batteries. On the DC Electrical page (click here) we have discussed the investigation into switching to a single battery voltage and how we came to remain with 2 voltages. We could have still switched the bow thruster batteries to Gel Cells and had a standard charing regiment on the boat. However this would have cost us in replacing the two chargers dedicated to the bow thruster batteries as neither could be switched to the charging voltage for Gel Cells. We also could have switched to AGM cells which offer higher current output than Gel Cells but again this would have required us to replace the chargers.

So we ended up staying with 24 volts and staying with wet acid batteries. The joinery in the forepeak was fitted when the bow thruster was installed and enclosed the batteries on all sides. With the four 800 CCA batteries we had a total capacity of 1,600 CCA at 24 VDC and wanted to replace as near that as possible. Re-working the joinery would have been a lot of trouble so we preferred to have direct replacements.

<more to come>

  • it is worth shopping around, we found our batteries for less than 50% of what was considered a competitive price within marine suppliers
  • we probably spent the most time researching this topic on the internet, prices, opinions, articles, etc.
  • a spreadsheet holding the option and prices is worthwhile as batteries are heavy and we found a number of “cheap” outlets made everything back on the freight charges (and funny enough some retailers who listed free shipping refused to include batteries no matter how many you ordered)

After a year the batteries are working well and behaving as the day they arrived. So far it seems that our sizing and configuration have been accurate.

After 2 years the battery for starting the engine failed to keep a charge. We contacted the manufacturer (Victron) to enquire about the 5-year warranty and were informed that we had to return the batteries to original point of purchase. The fact that we were halfway across Europe on the Danube river meant nothing to Victron, they would not let us exchange them at a local dealer. The cost to ship the failed battery to the original dealer was more money than buying another battery (we’re sure Victron knew this all along). So we replaced the battery – and it wasn’t with a Victron battery.

© The contents of this site are the copyright property of the authors.  Visitors may read, copy, or print any material for their own use, free of charge.  No material printed or copied from this site, electronically or in any other form, may be sold or included in any work to be sold without explicit permission from the authors.