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Old 23-12-2020, 06:23   #1
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FireFly Battery Notes II

I think the original Firefly operational notes thread is now too long for any new info to get read so:

It seems the recovery procedure with it's 0.4C charging rate is causing a lot of confusion. So I started a series of emails to Ocean Planet Energy who is the US Distributor and who I believe wrote the "manual" with that charge rate.

Question
The manual says to use a 0.4C charge rate. It seems my charge acceptance drops to only about 15 amps/G31 at a state of charge of about 85%, is this normal?

Answer
"a charge current tapering to 15-20A per battery could happen as early as 85% SOC so your observations show that it might be tapering a bit early but not dramatically so"

Question
I doubt most cruisers/ boats can do the 0.4C amp recovery charge for a house bank of G31s, and off grid cruisers have no practical way of taking batteries out of the bank to do as individual batteries. So what is the other method hinted at in the instructions? Is the high rate charge even needed since the batteries only accept that at low soc anyway? Or is the real factor the finish amps of 0.5/G31 at 14.4V absorption the key item, like other batteries?

Answer
"Unfortunately, the answers are not always totally black and white. We do encourage folks to upgrade at least one of their charge sources if they are getting FF so that a minimum of 0.2C charge current can be achieved. Much of our business is providing upgraded charging systems (upgraded alternators, etc) to satisfy clients demanding faster charging and reduced fuel use. Anyhow, the original Firefly inventor has told us that fast charging does improve the electrochemical processes inside the battery. However if after a deep discharge the finish amps as you mention are achieved, then perhaps as low of .2C for the bulk charging phase would suffice. Hard to say, as it isn't an exact science, unfortunately. What we DO know is that with fast charging of around .4C, the Firefly can retain/recover their capacity after many PSOC cycles, even without reaching the low finish amps (.5A tail current). In both our own and in 3rd party PSOC testing of discharging to 11.7V then charging for only 1hr starting at .4C, that after 30 such PSOC cycles the Firefly could be restored to full capacity by a couple full charge/discharge cycles. Whereas other AGM's suffered permanent capacity loss that could not be recovered.

Question
A lot of confusion exists in the boating cruising over this “recovery” procedure of the 0.4C. I feel this is a recovery procedure for trying to get back lost capacity if needed and not an everyday needed thing. I feel that for us charging primary with solar that acceptance amps at the 14.4v absorption is more important. Others really stress and feel it means to try to charge at this rate all the time (which of course means starting from low state of charge all the time) - so which is correct?

Answer
"We do know that the fast charging is beneficial, as it has proven effective when not quite getting to a full charge for extended periods (as in PSOC cycling).

It is certainly not required to charge at the .4C rate all the time.
It also isn't necessarily strictly applicable to the recovery procedure.

And yes, if you're not doing deep discharge cycles, then fast charging may indeed be less important than getting to the low acceptance amps,
And anyhow, at a low DOD there may not be much time spent @ .4C before hitting 14.4V.

However, what we don't know is what the possible detrimental long-term effects may be from:
1) Never doing periodic deep discharge/recharge cycles, or
2) Only charging at low rates after deep discharges.

There are many scenarios that testing could be devised for. All would take time and planning to get relevant results.

In any case, we recommended to occasional deep discharging/recharging, not only for conditioning, but also for capacity testing.
This would help track the effects of whatever use pattern the batteries are doing, and a baseline to compare with if subsequent recovery cycles are done."

If you read between the lines, because it is hard to get a straight answer:
1 - The 0.4C is a "recovery" charge not an everyday time.
2 - A "regular" charge rate of 0.2C is recommended
3 - Even the 0.2C can not be expected above 85% state of charge
4 - The finish amps of 0.5A at 14.4V is recommended to get to occasionally (note that it used to say 1.5A)

For me, I consider my FireFly batteries to basically be AGMs with added features of the carbon fiber grid that gets around the PSOC problems. But all the other normal charge rules I have learned still apply and it is better to have shallower discharges and to get fully charged based on absorption acceptance amps. If you do appear to have lost capacity you discharge them down and do the recovery procedure a few times to get it back (for me on the boat this would require taking batteries out of the bank to do).

I truly do not feel anyone knows and there has not really been testing that matches boat cruiser usage. Just like every other battery you need to read between the lines an apply the "manual" in a practical way.
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Old 23-12-2020, 08:39   #2
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Re: FireFly Battery Notes II

Another item that suggests that regularly dropping into the low SOC to where this 0.4C can be accepted isn't needed is the G31 tech sheet

DOD 30% - life = 9,000-13,500 cycles
DOD 50% - life = 3600-4500 cycles

Dropping down to a DOD so batteries can accept the high current shortens the life. This mirrors every acid based battery I am aware of.

Given that only 0.13-0.17C can be expected at 85% (15% DOD) just how much time would one stay at the 0.4C? I don't think I have ever seen more than 0.25C going into my bank and that was when they were down around 30% DOD/70% SOC and I was running my engine + battery charger + solar at full sun and the batteries still didn't accept all the current those could provide.

Has anyone actually done a "recovery" charge for a low SOC and can provide acceptance current data over time?
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Old 23-12-2020, 08:50   #3
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Re: FireFly Battery Notes II

Slightly ot, and apologies, but threads like this are extremely interesting to me as I'm anticipating electrical system upgrades pretty quickly after I get my boat next month.

My version of Calder's Boat Owners Mechanical and Electrical is 3rd edition, 2005. I just read the relevant chapters last night, no mention of Li at all. Zip.

Does the 4th edition provide good coverage of the updated technologies, or can anyone provide a good reference to complement what I already have? (FWIW Casey's book which I read cover to cover last week doesn't have Li coverage either)
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Old 23-12-2020, 10:17   #4
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Re: FireFly Battery Notes II

Here is a full discharge/recharge cycle for a new set of Firefly batteries done to set a baseline for longterm battery capacity and performance.

Overall bank C20 capacity is 464 A-hrs, 8 G31 making a 24V bank.

Discharge down to 21.5V at C10 rate of ~50Amps (+/- 2). Total A-hrs delivered was 448.6, essentally identical to the C10 rating for these batteries.

Initial charge rate of 135 Amps is 0.3C from a WhisperPower M-GV2 DC generator. What looks to be decreasing charge acceptance from 0% to 80% is not, but rather is the result of the DC genset being a constant power device, and the available Amps decrease as the voltage increases. If I really needed to get to 0.4C charge rate, I could by running shore power charger in parallel with the genset and my total charge rate would be close to 200 A.

At this charge rate (123 Amps), absorbtion voltage (28.8V) is reached at ~82% SOC.
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Old 23-12-2020, 10:37   #5
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Re: FireFly Battery Notes II

Thanks for the pass through on the discussion with Ocean Planet. The response lines up pretty well with the performance we see. With an 800ah bank we have to be on shore power and running the engine to see 0.2C / 160 amps.

I'll settle for a recovery charge the next time we are on a dock. That will likely be in July.

Cheers, RickG
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Old 23-12-2020, 10:41   #6
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Re: FireFly Battery Notes II

I pretty much what I understand have the same data from watching. Till about 82-85% SOC the batteries can accept all of a 0.15-0.2C charge rate and then it drops. at the that SOC my charger has more capacity than the batteries can accept. I have no idea, and don't believe your graphs show, if they could accept a lot higher and for how long rate. I don't know where in the DOD the batteries will accept the 0.4C.
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Old 23-12-2020, 12:18   #7
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Re: FireFly Battery Notes II

The most I've seen going into our new bank of six G31 FF from solar is 40 amps. Unfortunately, a solar charger (both are Victron MPPT) drops into float by early afternoon when the voltage hits 14.4 (or whatever the temp compensated value is) and the remaining charge controller sits at 13.5v putting in under 6 amps to the battery.

Both controllers ramp up output as needed to cover house load but I have no idea how to get them to stay at absorption voltage until they hit tail current.

They seem to sit at tail current but at float voltage.
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Old 23-12-2020, 12:50   #8
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Re: FireFly Battery Notes II

Someone please explain to me what .4c and .2c means.
Thank you!
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Old 23-12-2020, 14:03   #9
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Re: FireFly Battery Notes II

Quote:
Originally Posted by freshalaska View Post
Someone please explain to me what .4c and .2c means.
Thank you!
C = battery 20hr discharge rating in amp-hours (AH), for the FF G31 each is 116AH so 0.2C is 116x0.2=23.2amps
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Old 23-12-2020, 15:17   #10
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Re: FireFly Battery Notes II

Quote:
Originally Posted by crayiii View Post
The most I've seen going into our new bank of six G31 FF from solar is 40 amps. Unfortunately, a solar charger (both are Victron MPPT) drops into float by early afternoon when the voltage hits 14.4 (or whatever the temp compensated value is) and the remaining charge controller sits at 13.5v putting in under 6 amps to the battery.

Both controllers ramp up output as needed to cover house load but I have no idea how to get them to stay at absorption voltage until they hit tail current.

They seem to sit at tail current but at float voltage.
Your solar controllers have no direct way of knowing what the tail current actually is because they can not know what amount of power they produce is going into the batteries and what is going out to loads.

On my system the constant load averages 200 Watts, but varies from 50 to 300. Batteries are fully charged when they are accepting about 150 watts. There is no way for the solar controllers to know what fraction of the power is going to battery or load, hence they can't use current as a target for ending absorption.

Victron's controllers run their absorption cycle out to a constant time based on their best guess for the batteries' state of charge. They guess that by looking at the voltage when they first wake up in the morning.

I get around this by setting my solar float voltage to be the same as my absorption voltage. There is very little or no opportunity for the solar system to significantly overcharge my batteries, so it works for me. Your mileage might vary.
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Old 23-12-2020, 15:41   #11
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Re: FireFly Battery Notes II

Quote:
Originally Posted by sailorboy1 View Post
I pretty much what I understand have the same data from watching. Till about 82-85% SOC the batteries can accept all of a 0.15-0.2C charge rate and then it drops. at the that SOC my charger has more capacity than the batteries can accept. I have no idea, and don't believe your graphs show, if they could accept a lot higher and for how long rate. I don't know where in the DOD the batteries will accept the 0.4C.
If you looked more closely you would see that I am charging my batteries at 0.35 C for most to the way from 0 to 82%.

The charge acceptance has nothing to do with "how long" they have been charging, but is only controlled by SOC. In other words, the more charge current you have, the lower SOC will be when they reach absorption voltage, although it is not at all linear.

For my battery bank, they will hit absorption voltage at 90% SOC at 0.1C charge, 85% SOC with 0.2C, 82% at 0.3C and 80% at 0.4C. Never pushed higher than that. Once you hit the Absorption voltage the rest of the curve is identical no matter what the initial charge rate was. From 82% SOC to a tail current into the batteries of 0.015C takes about 150 minutes.

In normal operation, my system is set to cycle down to 70% SOC which normally takes two or three days. Then it automatically runs the genset to bring them up to 93%. Solar then tops them off. This results in about 40 minutes of 0.3C charge before it reaches 82% SOC, then another 45 minutes of decreasing charge rate.
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Old 23-12-2020, 15:48   #12
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Re: FireFly Battery Notes II

It occurs to me that all I wanted to do was put the info out there and I, at least currently, aren't feeling a need to discuss or overthink.
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Old 23-12-2020, 20:25   #13
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Re: FireFly Battery Notes II

Quote:
Originally Posted by sailorboy1 View Post
It occurs to me that all I wanted to do was put the info out there and I, at least currently, aren't feeling a need to discuss or overthink.
If all you wanted to do was pontificate and let everybody know how smart you are, you shouldn't have asked for contributions of information...

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Has anyone actually done a "recovery" charge for a low SOC and can provide acceptance current data over time?
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Old 24-12-2020, 04:14   #14
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Re: FireFly Battery Notes II

Quote:
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It occurs to me that all I wanted to do was put the info out there and I, at least currently, aren't feeling a need to discuss or overthink.
Wow. You're something else.
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Old 24-12-2020, 05:58   #15
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Re: FireFly Battery Notes II

I put the info out. Direct quotes from the guy who wrote “the manual”, Sorry if people now get get wound up that I have decided to not continue a back and forth argument about it. You do whatever you want, it ok with me by far.
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