Paul Osterberg

We need new batteries in our SM, the last (sealed) batteries lasted less than 2 ½ year (No we never run them below 70% and often charge them close to 100% most days as we have 850 w of solar power so they have not been miss treated).

 I’m thinking on the FireFly OASIS AGM batteries, if half what the producer says is true they should be an excellent choice. They are rather expensive 486 $ plus tax and freight for a group 31 typ.

Has anyone any experience with them?

Another option is LiFePO4 batteries I found from Smart Battery 100 Ah group 31 for 1300 $

4 of them would give  more usable capacity at a cost of  5200 $ compare with 3900 $.

The Smart Battery come with 5 year’s replacement warranty and a pro rata life time warranty. They claim you can use your standard chargers, so no other investment needed.

But I assume with the strong focus on electric cars the development of batteries will be huge the coming years, so maybe better to wait to install LiFePO4 batteries until next time.  


Paul on SY Keepa SM 259 



In September 2017 we purchased 12 x Deka Marine Master flooded G31 batteries. If you are still in the Annapolis MD area, I strongly reccommend that you buy them from Stevens Battery Warehouse in Annapolis. (

We paid US$120 for each battery and with sales tax the total came to US$1624 for all 12 batteries. The staff is super friendly and they will load test each new battery right in front of you so you know it's good. They have them in stock by the pallet and seem to turn their inventory over quickly as the batteries we purchased in September had stickers saying they were manufactured in July 2017. They also carry the Deka AGM Intimadator batteries as well as Lifeline brand batteries.

Mark McGovern
SM #440
Deale, MD USA



You asked for comments on experience with Firefly batteries.  I don’t have any yet, but thought I would share our logic on our recent battery purchase.

First, I agree 100% with Bill Rouse that if your primary motivation is keeping the annual cost of batteries as low as possible, it is hard to beat flooded lead acid batteries. They do have a short lifespan, but are also really cheap, quite robust, and easy to get. Bill R’s point about making sure your charging system can match the charging curve of any kind of battery you pick is also critical.  This is a place where “close enough” is not close enough.

I do not think that Lithium batteries are quite to the point—yet—of being suitable for remote cruising.  They still seem fussy enough that I’d really want to be close to technical support if I had them. It is also likely that your engine alternator can not charge them.  Not because of voltage mismatch, but because Li batteries can accept full charging amps for almost the full charging cycle.  Very few alternators can supply their fully rated current for the time required to charge a Li battery.  The alternator will overheat and die. Alternators rated for continuous output at full rated current are rare and very expensive. Li batteries are probably the batteries of the future, whenever that gets here!

Harmonie has six year old Lifeline AGM batteries currently installed. The are just coming to the end of their useful life, with a capacity of about 60% of new.  On an annual basis, that puts them at almost exactly the same cost as flooded cells.  For those six years we have enjoyed not adding water, and a higher charge acceptance rate, so I would say the net effect is a positive, although not a huge one.

After much thinking and back and forth on the decision, we have decided to replace our aging AGMs with Firefly batteries--but not because we expect them to be cheaper than the alternatives.  My expectation is that they will last 3 to 4 times what a flooded cell bank would, so the amortized annual cost will be similar.

The reason we went with Firefly is because of their tolerance for extended periods at partial charge. They do not need to be regularly brought to 100% charge to live a long and happy life. This matters to me because my solar panels are capable of making 3.5 to 4.0 kW-hrs of power over the course of a sunny tropical day.  This should be enough to run the whole boat, but we can not currently store all that power.

We can not store that solar power because the batteries can not accept it.  We try to refill our AGM batteries as close to 100% every day possible. Once the they move out of the Bulk charge phase and into Acceptance (at about 85% Charge) the controller ramps back the power from the panels.  If we want the batteries full, we need to do the bulk charge in the morning with the generator, and let the solar system slowly top off the batteries over the course of the day. This means we only get to use only about half the power the solar panels could have produced.

With Firefly batteries we expect to run the batteries from 50% to 80% charge every (sunny) day using just the solar panels, and skip the generator--most days.  Our generator is getting old, with 7000+ hours.  If I can put off buying a new genset for a year, I will have paid back the premium price of the Firefly batteries.  My objective is to have a future owner of Harmonie be the one who has to buy her next genset.

We have a fully programable lines battery charger and solar charger, so we can get whatever charge curve we need.  The Fireflies charge at 14.4V (28.8V) which is the same as many flooded batteries, but if you are plugged in at the marina, or doing extended motoring, they want to float at 13.2V (26.4V) or LESS which is lower than other types of battery.  You need to be sure you can supply this lower voltage.

One significant drawback of the Firefly batteries is the availability is very tight.  Nobody has them in stock, and deliver schedules are spotty.  We ordered ours in August, were told that they would be available in October, and they are finally being delivered here in December.  Don't expect delivery right away.

Is my logic sound? 
Are the manufacturer claims and other testing of the Firefly batteries accurate?  
Will these batteries let me greatly reduce my generator run time without impacting battery life?  

I’ll get back to you in five years.  Or sooner if I am wrong!  

Bill Kinney
SM160, Harmonie
Fort Lauderdale, FL

Paul Osterberg

Thank you Bill!

As we only going to sail about 6 month/y the coming years I think the firefly make sense to us as well, we do not need to worry about leaving the batteries for a longer period without charging. I also believe it is added value with a battery that last for longer time. Did not know about the 26.4 v charging for long time, but we have a dumb regulator and will try to find a smart regulator who can handle that.

Paul on SY Kerpa SM#259


Paul Osterberg


Thank you for the info, good to know

Our boat is in Georgetown, Sassafras River, we will be back aboard mid/end April if things goes as planned

Paul on SY Kerpa