Discharging AGM batteries beyond 50% - could it be a smarter choice?


The Credible Hulk
Interesting. But for some reason none of their listed applications are automobiles...

General info from their website http://fireflyenergy.com/

Sealed – requires no maintenance
More than 3-4 times cycle life compared to flooded gel & AGM VRLA batteries at 50% DOD.
Lower cost per kWh delivered compared to premium VRLA batteries.
Unmatched ability to recover from extended storage in discharged state.
Throughput efficiency greater than 90%.
Improved high/low temperature performance.
Superior protection against corrosion and sulfation related problems.
Available with Battery Energy Management System (BEMS) .
Lowest Cost of ownership
Industry leading warranty.
Compatible with existing lead acid battery recycling infrastructure.
Outstanding long life even under partial state of charge operation.


One thing that is often missed in battery bank sizing is what is your biggest load?
FLA batteries should not be drawn down at a rate higher than 25% of capacity, for AGM it is 30%( one of the few ares AGM is better than FLA).
So if you run a heavy appliance such as a microwave and it draws 125 amps you need a 500amp/hr bank for FLA.
I believe you can load Lithiums as much as you want.


Yeah, here in North America, (well actually there in North America I am still in Russia) the best place to go for house rig electrical is marine. If it works in a boat it is more than adequate for expedition use. Hence why you see so many people running BlueSea isolators or charge control stuff now. Before it was the 800amp can style relay isolator. What I like about these is standard group 31 size and the ability to handle partial state of charge for long periods. Reading a few different articles they seem to take a charge faster, and don't need a float charge. The real bonus in my mind is you don't have to oversize for the SHTF moment. If you need 200 Amp/hr you can buy 400 amp/hours of battery 50% and if you get a SHTF event you have the reserve 100 before "potentially" damaging the batteries. These can be resurrected quite easily from what I've read. The fact that these are recommended by one of the original Gurus of lithium systems for boats made my decision easy.


Expedition Leader
Some Comments ...

NOT backed up with personal experience.

A local marine electrician, who is well regarded, loves the Firefly batteries. They were developed by Caterpillar, but are made in India. Quality has been an issue with some batches.

He noted:

-- Sailors often prefer a lead acid battery to a lithium battery because they want the weight for ballast. (A marine anti-sway bar, if you will.)

-- As sailors often have less space for solar, they have a harder time achieving a complete recharge, even with a generator. (A genset is a very inefficient way to do the absorb charge stage as the fuel consumption doesn't drop that much, even if the power output does.)

And, as we all know, failing to fully recharge a lead acid battery is a prescription for early failure.

So, a heavy battery, that allows a deeper discharge, is not damaged by a partial recharge, and uses a conventional charging profile is very attractive. A lot more power in the same space/weight.

Hot ticket for overland vehicles? I have no/no personal experience, but if I had a smaller vehicle where space and weight were big issues, I might give it a shot.

As always, YMMV!
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Tail-End Charlie
Also much wider usable temp range than lithium, and more cycles lifespan than other lead-acid types (at partial discharge...at full discharge, about the same).

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I agree the FireFly looks interesting. My biggest question with it is: how will it react to real world charging conditions in vehicles? According to FireFly, their battery wants bulk charge at 14.4v and no float charge (but 13.4 is okay if you must). They don't mention an absorption voltage. What does your alternator put out? Maybe pretty close to this but maybe not (newer vehicles do all kinds of crazy things - my 2010 Frontier varies from 12.7v to 14.7v).

Their impressive test results all seem to be on bench chargers (presumably set to a steady 14.4v). Maybe these batteries are tough enough to handle the non-traditional charge profiles found in many newer vehicles (engineered for flooded lead acid) but I don't think there is much evidence either way yet. If you think all lead acid batteries charge the same, look in to gel batteries - folks charging these with flooded lead acid charging profiles found they didn't last long at all.

Will be eager to see some real world experience as people start using the FireFly product and (hopefully) reporting.