LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

[NahanniV]

Originally Posted by diugo

I really like the idea of a plastic cover over the battery terminals, but why polycarbonate? Why not a softer plastic such as PVC or HDPE?

Probably so he can see the LEDs on the BMS modules without removing the cover.

Maine Sail:

Have you spoken with CleanPower about the cell modules ?
I don't see a 400Ah size listed, guess it would just be a matter of extending the leads.

[NahanniV]

Never mind, I see them now

[diugo]

I just got my UPS tracking number from Balqon! In theory, the batteries should get picked up on Monday and arrive Wednesday.

[deckofficer]

Good to hear Doug. I'll be curious as to the cell dates. I'm cycling mine now with a 30 watt LED light load. After 22 hours of draw, no cell is any further than 0.07 volts from each other, and I'll bottom balance before the next charge. After 660 w-hr draw, the cells are 3.27, 3.27, 3.20, 3.23 volts.

[Lagoon4us]

We are underway with prepping our boat to eventually fit our cells in March, solar will be up and mounted, monitors in place etc.

I have probably a naive question but to counter the possibility of supply too high a charge voltage to the cells is there a dc-dc transformer or whatever that only allows 13volts or whatever to the cells? Like a choke, adjustable?

I realise the regulators are sensing and throttling back their individual sources but in the situation of surges, broken regulators or bad luck can't a tranny be wired to only give the req volts therefore protecting from overcharging in general?

Cheers apologies if a dumb question just trying to get a useful understanding.

[deckofficer]

Others are better versed than me on this subject. When you get your new cells balanced, if needed, and it is a 12 volt system, I would set chargers, alternators, and solar controllers for 14.8 volts, which works out to 3.7 volts per cell. If not using a BMS, you have to manually balance the cells in your 12 volt pack and check on a regular basis. I'm just starting to do that now to my cells. I'm amazed at how little the voltage drops under my big single cell load I'm using to balance, I'm so used to voltage sag of lead acid.

Winston states that their cells shouldn't be pushed past 4.0 volts during charging. If not balanced, this can happen during charging at 14.8 volts. That happened to me on my first charge of my 2nd bank. The first bank was balanced good enough that no cells reached over 4.0 volts. On the 2nd bank at about 90% charge, one cell was over 4.0 volts so I stopped charging.

[deckofficer]

I might have to email Balqon as I still can't get that one cell in pack #2 to balance. On this cycle, when the charger had dropped to 8 amps, 3 of the cells were around 3.55 volts and that errant cell was at 4.25 volts, way too high for decent life.

[s/v Thea]

Originally Posted by deckofficer

I might have to email Balqon as I still can't get that one cell in pack #2 to balance. On this cycle, when the charger had dropped to 8 amps, 3 of the cells were around 3.55 volts and that errant cell was at 4.25 volts, way too high for decent life.

Most likely the high cell is at a slightly higher state of charge, voltage increases very quickly as cell reach full charge. What you need to do is draw some charge out of the high cell using a resistor or other load. Start with 1ah, try charging again, repeat if required.

Looks like you are charging the pack to 14.9V, (3x3.55+4.25). Personally I would limit pack voltage to 14.4 or so.

[mcarling]

Originally Posted by Lagoon4us

I have probably a naive question but to counter the possibility of supply too high a charge voltage to the cells is there a dc-dc transformer or whatever that only allows 13volts or whatever to the cells?

A solid-state DC-DC converter will do that but you want one that is designed, built, and marketed as a battery charger with a specific setting for LiFePO4 batteries. It would output whatever voltage is appropriate depending on your State of Charge and accept DC input voltages from very roughly about 8V to very roughly about 20V.

[Maine Sail]

Originally Posted by deckofficer

I might have to email Balqon as I still can't get that one cell in pack #2 to balance. On this cycle, when the charger had dropped to 8 amps, 3 of the cells were around 3.55 volts and that errant cell was at 4.25 volts, way too high for decent life.

Bob,

Take the cells out of series and wire them in parallel and let them sit for a day or two. They should be fairly well balanced after this. Then charge them in parallel. You will need an adjustable power supply for this but at the cost of these cells that is cheap money.. You may need to balance again in the future.

[deckofficer]

Originally Posted by s/v Thea

Most likely the high cell is at a slightly higher state of charge, voltage increases very quickly as cell reach full charge. What you need to do is draw some charge out of the high cell using a resistor or other load. Start with 1ah, try charging again, repeat if required.

Looks like you are charging the pack to 14.9V, (3x3.55+4.25). Personally I would limit pack voltage to 14.4 or so.

I'm doing that and it didn't help. I'm using battery jumper cables as a resistor, shorting out the cell for a few seconds at a time (staying within the C surge rating's time frame) to pull the voltage down on a 70% DOD cell to match the voltage of the other 3 cells. When recharging as the 50 amp charger has tapered to 8 amps that cell was too high again.

[deckofficer]

Originally Posted by Maine Sail

Bob,

Take the cells out of series and wire them in parallel and let them sit for a day or two. They should be fairly well balanced after this. Then charge them in parallel. You will need an adjustable power supply for this but at the cost of these cells that is cheap money.. You may need to balance again in the future.

I'll do that after talking to Balqon because it will be a PITA to un-band the cells and run the buss bars in parallel, plus I still need to get a adjustable power supply.

[deckofficer]

Originally Posted by Maine Sail

Bob,

Take the cells out of series and wire them in parallel and let them sit for a day or two. They should be fairly well balanced after this. Then charge them in parallel. You will need an adjustable power supply for this but at the cost of these cells that is cheap money.. You may need to balance again in the future.

Instead of spending $275 for the 30 volt, 30 amp variable power supply that your using, any problem other than longer times with using a $50 18 volt, 3 amp variable ps?

[s/v Thea]

Originally Posted by deckofficer

I'm doing that and it didn't help. I'm using battery jumper cables as a resistor, shorting out the cell for a few seconds at a time (staying within the C surge rating's time frame) to pull the voltage down on a 70% DOD cell to match the voltage of the other 3 cells. When recharging as the 50 amp charger has tapered to 8 amps that cell was too high again.

If you want to bottom balance you need to draw down to almost 100% DOD or at least until there is a significant drop in voltage. The voltage curve is so flat that accurately telling 70% from 80% would be pretty difficult.

Also if you bottom balance you can't expect the pack to be balanced at the top unless capacity of all batteries is identical, which is unlikely.

I had similar situation with one of my packs, I was already on the boat for the summer and all I had was a 220 ohm, 5 watt resistor which drained about 1/2 ah per 24 hrs. Took almost a week to get the cell in balance, since then I have had no problems.

[deckofficer]

It is going the right direction over the first cycle below......

Pack #2 1 hour after a 1 hour charge (stopped early due to one cell going too high)

1st cycle 2nd cycle after some balancing
Cell 1 3.37 volts 3.34
Cell 2 3.51 volts 3.45 this is the cell that keeps jumping to over 4 volts
Cell 3 3.33 volts 3.32
Cell 4 3.34 volts 3.34

I only pulled an extra load from cell 1 and 2, seems to have helped ever so slightly.

I will cycle deeper on the 3rd cycle. The 2nd discharge was running a 30 watt load for 22 hours, so 660 w-hr from a bank of 1280 w-hr. I'll go for 100% DOD which should be 42 1/2 hours on that 30 watt load.