How do you drain Firefly batteries per manual?

[ChristineKling]

It turns out it ain't easy!

We made the decision to go with Firefly 4V batteries on our newly constructed boat, Möbius, about two years before the boat launched. In the interim, Firefly has revised their battery manual in the latest May 2021 edition. There is now this section on how to perform a "Restoration Charge."

Restoration Charge
As stated, Firefly Batteries can operate in a partial state of charge for long periods of time without sustaining any permanent damage. The usable capacity will decrease, however, with each cycle within a partial state of charge, up to a point. In order to regain the full original capacity and in some cases more, it is necessary to perform a restoration charge. To perform the restoration charge: charge the G31 to 14.4V or the 4V/450AH to 4.8V and continue to charge until the current drops to 0.5 A on the G31 or 3A on a 4V/450AH and apply a float charge of 13.5V(4.5V for 4V model) for 24 hours. Fully discharge the battery to 10.5V (G31) or 3.5V (4V/450AH), and then repeat the same charge cycle.

Turns out it is actually difficult to drain a huge battery bank that low (we have 24 of these batteries) on a complex boat, especially when living aboard. We are able to break our large 24V bank into two separate 24V banks and work at discharging one bank while living off the other, which might sound strange, but there is very little aboard that will continue to run at a low voltage. Wayne has been able to tweak the settings on the Victron inverters so they don't shut down quite so soon, but we need to get our 24V bank down to 21V! All the LED lights just aren't going to do it.

A few days ago I came back to the boat and Wayne was running the DC bow thruster back and forth. It didn't put a dent in the amps we need to dump. Now he's looking into putting an additional DC water heater element in our AC water heater.

Anyone have any thoughts about how to drain these batteries without damaging the equipment?

By the way, if you have Firefly Batteries on your boat, I've started a Facebook Group where we can share information about this and other Firefly issues. Please join. https://www.facebook.com/groups/899302057442444/

 

[twistedtree]

Perhaps another cheap inverter that supports a lower cut off voltage? Then use it to power a small AC heater until the inverter shuts down? It might cost more than adding a DC heating element to the water heater, but would probably be a lot easier to do.

I had to do this some time ago on a system, but the inverter supported a low enough cutoff voltage so it wasn’t too hard.

 

[O C Diver]

If you have an inverter, read the manual for low voltage limitations. Most will operate in or below the 10 volt range. This can happen when an inverter is under a heavy load and the line voltage drops. I wouldn't use a heavy load near the end, to drain the batteries, but a modest load should work.

Ted

 

[twistedtree]

If you have an inverter, read the manual for low voltage limitations. Most will operate in or below the 10 volt range. This can happen when an inverter is under a heavy load and the line voltage drops. I wouldn't use a heavy load near the end, to drain the batteries, but a modest load should work.

Ted

This is what I did some time ago, but I took Christine's post as an indication that their Victron inverters can't be set that low for the low battery cutoff.


But I just launched Victron's config app and it looks like it can be set well below 21V, so this does indeed seem like the way to go. Just set the LBC to 21V (or whatever you want), turn on loads, and wait for the light to go out.

 

[cafesport]

Christine,
I just had a quick look at firefly’s spec sheet and it says you should be able to discharge them in 5 hours using a 20 amp continuous load. The problem is you’ll need to further isolate the battery bank into pairs to accomplish this. Like eating an elephant think small chunks. Using an inverter load won’t work unless it is constant for the specified time interval as per firefly’s specs. You need a proper load bank and a timer. Doing a whole bank of 12 batteries at one time could also be problematic. Our house bank consists of 8 batteries in series and paralleled to form 4 independent isolatable 24 volt banks. For load testing and equalization we simply isolate one bank of batteries after a full charge and then do a discharge at the 10 hour rate as per the manufacturers spec.

 

[hollywood8118]

Why not get a couple of the cheapest large incandescent old school headlights, wire them up with a couple alligator clips and carefully hang them ( they will get hot!) and let them go to town on the batteries.

HOLLYWOOD

 

[RT Firefly]

Greetings,
Well, I don't know about your situation but a cheap bottle of rye and a fast blonde does it for me....

 

[Hippocampus]

The way to dump excess electricity with solar is a few old school resistors inside a heat shield. Had one off the shelf for each D400. Know that’s several orders of magnitude smaller then what you need. But for a few bucks would think you could McGiver something up just larger scale. Attached to a board and placed outside it could be done safely.

 

[Comodave]

Why not get a couple of the cheapest large incandescent old school headlights, wire them up with a couple alligator clips and carefully hang them ( they will get hot!) and let them go to town on the batteries.

HOLLYWOOD

This is a good idea. Keep a fire extinguisher handy though.

 

[ChristineKling]

Perhaps another cheap inverter that supports a lower cut off voltage? Then use it to power a small AC heater until the inverter shuts down? It might cost more than adding a DC heating element to the water heater, but would probably be a lot easier to do.

I had to do this some time ago on a system, but the inverter supported a low enough cutoff voltage so it wasn’t too hard.

Thanks for this good suggestion. We've been looking into this, but most of even the cheap inverters we're seeing boast of their automatic cut-offs at low voltage for self protection.

 

[twistedtree]

Thanks for this good suggestion. We've been looking into this, but most of even the cheap inverters we're seeing boast of their automatic cut-offs at low voltage for self protection.

It's usually adjustable, as the Victrons are too. Is it not possible to just lower the cut off voltage setting on your existing inverters and run them until the shut off at the prescribed voltage? I think you were looking for 21V, and from what I saw, that's doable with the victron inverters. Or no?

 

[cafesport]

There was a pretty good discussion of these batteries on the boat electrical systems group on Facebook over the summer.

The batteries do sulphate if not charged at .4C and will loose capacity over time, but they are fully recoverable. To do a restoration charge what works best is to discharge them at the 20 hour rate until the voltage reaches 21 volts under load, then immediately do a recharge at .4C.

Do this twice and you should be back to full capacity. For your bank it looks like your banks are 900 ah at 20 hours so that would work out to 540 watt load for 20 hours.

You could build a 1.067 ohm resistor and use alligator clips similar to this


or plug in a 500 watt halogen work light, assuming the inverter consumes the other 40 watts in the process. According to people who have done this it will take two complete cycles to get you back to original spec.

 

[Whodgins]

Thanks very much for all these suggestions and most of them point me to where I've been going so that is reassuring. Below are some additional details to answer some of the questions and outline what I've done so far. All critiques and additional suggestions welcomed and encouraged!

Our large 1800Ah @ 24V house bank is made up of 24 FF 4V cells which I wired 6S4P, so there are four independent 24V @ 450Ah banks. I installed battery switches so that each of the 4 banks can be isolated, which is proving to be very helpful in fully discharging these batteries down to <21V in order to do the .4C Restoration Charge.

Last week I used Victron Connect to create a custom configuration setup file for our 240V Victron MultiPlus 5000's that sets the LV cutoff to 20V (this is a 24V setup) and this is working as I had hoped and as Peter and several others suggested (thank you), so I think I have that sorted out now. This allows me to put the required load on one bank at a time and keep the other 2 or 3 banks online to power the boat as per normal.

However it isn't practical to use an AC load to drain that one bank of batteries because we need the inverters running off the other banks to be running the rest of the boat's system as we are full time live aboards and need various day to day AC equipment for cooking, interior heat, etc.

Based on my research into the updated requirements for FF batteries, I will need to do this Restoration Charge several times a year so I am also trying to create a solution which is relatively easy to execute repeatedly. My current plan is to permanently wire in a 24V DC "dump load" out of some combination of a 24V heating element in our IsoTherm calorifier and/or several dump load resistors same as CafeSport's suggestion (thanks!) and photo above. I had found these in my research but so far have not been able to find any source for these here within Turkey and getting things shipped into Turkey is very time consuming and quite the PITA. But looks like I'll just need to go that route which we've got lots of experience with, just not very good ones!

The plan then is to be have this large DC Dump Load that I can selectively connect to one isolated 24V bank of FF batteries while keeping the other 3 banks connected as they normally are to our house loads and inverters.

Make sense? Better ideas?

I'm very open to any and all critique, concerns or improvements to this plan or any other helpful suggestions from those with any experience doing these Restoration Charges on FF batteries.

Again, thanks for all of you help so far and once I have a solution in place and have done two Restoration Charges and hopefully restored them back to original capacity I will report back to you all here.

-Wayne
Christine's not so better half

 

[DDW]

This thread is an excellent advertisement for LiFePo4 batteries.