LiFePO4 woes in Caribbean

[toddedger]

Garyh Conventional wisdom says you batteries would be a lot better off running between 50% and 60% then spend there life between 80% and 90%. I'm not sure how you determine where your at and when you start charging, but if your only going to cycling 10% then your really just in storage mode.

[Drelen]

I have a similar problem with a Balqon 700AH battery. When my solar fully charges the batteries, three cells are even and one is just a bit lower. After using 180AH, sometimes my residential refrigerator goes into defrost, putting a 50A land for 20 minutes. This can cause the battery pack to shut off. When it does, three of my cells are 3.32, and one is 2.85 to 2.75. When I turn the battery back on it reads 12.8V. I was planning on replacing the bad cell, but after reading this posting I wonder if the bottom balancing worked for the long term? Also what type of charger do you use to charge just one cell? Thanks

[kmacdonald]

Originally Posted by Drelen

I have a similar problem with a Balqon 700AH battery. When my solar fully charges the batteries, three cells are even and one is just a bit lower. After using 180AH, sometimes my residential refrigerator goes into defrost, putting a 50A land for 20 minutes. This can cause the battery pack to shut off. When it does, three of my cells are 3.32, and one is 2.85 to 2.75. When I turn the battery back on it reads 12.8V. I was planning on replacing the bad cell, but after reading this posting I wonder if the bottom balancing worked for the long term? Also what type of charger do you use to charge just one cell? Thanks

VoltEQ DC power supplies work good to charge single cells or the entire pack.
Volteq variable DC power supply - Volteq - Reliable Regulated Variable DC Power Supplies

[Drelen]

Will the $65 Volteq charger do the job? The rest are pretty expensive.

[kmacdonald]

Originally Posted by Drelen

Will the $65 Volteq charger do the job? The rest are pretty expensive.

Get this one for $140: Variable Switching DC Power Supply HY1520EX 0-15V 0-20A - Volteq - Reliable Regulated Variable DC Power Supplies
It is on the small size for a 700 Ah battery but will work if you are patient. It can charge single cells or the entire pack.

Or this one for $190: http://www.volteq.com/volteq-power-s...rotection.html

[Gary H]

Update:

After more than a month, everything seems to be OK. I did not actually "top balance" the cells due to time constraints at the dock, but did get the weak cell up to approximately full using a bench top power supply. Since then, I have stayed away from the knees on the charging curve and voltages have remained within 0.02V across all cells under normal usage.

In a couple of months once I am back in the US, I will do a proper balance and a capacity test to see where the cells really stand.

Hope this helps.

Gary

[Drelen]

I ordered the Volteq-power-supply-hy1520ex-15v-20a. When it comes I will teach my low cell a lesson. Thanks for all your help.

[Drelen]

A question about single cell charging which I have never done. (A) Do I need to disconnect the cell from the rest of the battery before I charge it? (B) Can I leave the battery connected up and just put the charger leads on the weak cell? (C) Can I leave the battery connected together, attached the charger to the weak cell, and power the charger through my inverter. This would seem to draw down the strong cells and charge the weak cell, bringing them into balance sooner. I have experience where my brillient ideas are not so brilliant, and I would prefer to not wreck my new charger or my battery. Any advice would be appreciated.

[kmacdonald]

First off, turn on the charger and adjust the voltage BEFORE connecting to the battery and disconnect the battery BEFORE turning off the charger.
If you disconnect the battery from the load you don't need to disconnect the cell from the battery. Don't do B or C.
I would charge it at 3.45V per cell. DO NOT OVERCHARGE!!!!!!

[Drelen]

If I understand you correctly, I flip the switch that totally disconnects my battery from my system. Then I turn on my new Volteq battery charger and adjust the voltage, preferably to 3.45V. Then I can connect the charger leads to the weak cell of the battery and charge it without disconnecting it from the rest of the battery. When I am done, I disconnect the charger leads from the battery BEFORE I turn off the charger. If this is not correct, let me know. Otherwise I will follow these instructions. Thanks

[kmacdonald]

Originally Posted by Drelen

If I understand you correctly, I flip the switch that totally disconnects my battery from my system. Then I turn on my new Volteq battery charger and adjust the voltage, preferably to 3.45V. Then I can connect the charger leads to the weak cell of the battery and charge it without disconnecting it from the rest of the battery. When I am done, I disconnect the charger leads from the battery BEFORE I turn off the charger. If this is not correct, let me know. Otherwise I will follow these instructions. Thanks

That is correct.

[poiu]

I don't think Kmacdonald is correct that you need to disconnect. I charge a single cell from an inverter fed power supply whilst the pack is all connected up. It works fine.

[kmacdonald]

Originally Posted by poiu

I don't think Kmacdonald is correct that you need to disconnect. I charge a single cell from an inverter fed power supply whilst the pack is all connected up. It works fine.

If you can explain how you can determine when the battery is fully charged with it still connected and drawing loads, I'd like to hear it. A battery is considered fully charged when it accepts 0.5% of it's Ah capacity. A 100 Ah battery is fully charged when it accepts 0.5A at 14.6V. (typical for a lead acid battery)

[poiu]

Originally Posted by kmacdonald

If you can explain how you can determine when the battery is fully charged with it still connected and drawing loads, I'd like to hear it. A battery is considered fully charged when it accepts 0.5% of it's Ah capacity. A 100 Ah battery is fully charged when it accepts 0.5A at 14.6V. (typical for a lead acid battery)

You don't need to fully charge it to deal with his imbalance problem and I think that would be wrong to do.

A top up at the bottom works well. You can easily estimate how much charge is out from the difference in voltages, which at the bottom correspond to state of charge quite accurately. So for example a 45Ah imbalance can be dealt with by running down the main pack at its average draw of say 10A and charging the bad one up at 5A and doing this for 3 hours gets you close to balance. It's quite easy.

[kmacdonald]

For LiFePO4 voltage is not a good SOC measuring tool. To balance the cells correctly they should all be fully charged and then connected in parallel for a period of time. A wet finger in the air won't get it done to any degree of certainty.