Can I use a car battery charger?

[KiwiMark]

I tried getting down to 8.1v, but the charger only discharges at a minimum of 0.1amp. The pack couldn't even sustain this draw without dropping to 1v. Once the draw was removed the pack went back to 9.5v. What is going on?

If the pack drops to 1V under a 0.1A load then pretty much every cell in the pack is completely flat - time to charge that pack!

 

[poormanq45]

Well, 1 V is way, way lower than 8.1 V, isn't it? Doesn't the charger automatically stop the discharge at the voltage you set? How does it even get down to 1 V?

For a pack with many cells it is often recommended to discharge to 1.0 V per cell, or even 1.1 V per cell. This is the voltage under load. As soon as you remove the load the voltage should gradually rebound to about 1.2 V per cell. If it doesn't, the cells are damaged.

Remember that deep discharge of a pack is potentially damaging. Don't try too hard. Discharge down to 9 V and stop, don't keep discharging further.

I was manually discharging because the protection circuit kept kicking in. The charger takes the settings that you input and uses them even under a dynamic load as a stopping point. When I let the charger stop at 8.1v the pack stabilized at 10.8v!

The voltage dropped to 1v under load. When I removed the load the voltage stabilized at 9.5v after about 20 minutes.

Well, I just recharged the pack at 0.3amps for 12 hours, which is the longest the safety will allow without going in to the settings and disabling the feature.

The pack stopped at 13.52v. Input charge of 3412mAh. Temperature was around 100 degrees. I'm not sure if the termination signal was missed or not because the safety time limit was hit.

 

[Mr Happy]

I was manually discharging because the protection circuit kept kicking in.

That isn't the protection circuit. That is the "stop discharging when programmed to stop" circuit.

The charger takes the settings that you input and uses them even under a dynamic load as a stopping point.

Exactly.

When I let the charger stop at 8.1v the pack stabilized at 10.8v!

That's exactly what is supposed to happen! Because 9 x 1.2 V = 10.8 V. A voltage of 1.2 V per cell is the lowest resting voltage you ever want to see on your pack. If you go lower than that you are damaging it.

The voltage dropped to 1v under load. When I removed the load the voltage stabilized at 9.5v after about 20 minutes.

Right. When the batteries are fully discharged they cannot supply any more current to a load. So if you try to apply a load the voltage decreases to towards zero. This is an important sign for you to stop discharging.

Well, I just recharged the pack at 0.3amps for 12 hours, which is the longest the safety will allow without going in to the settings and disabling the feature.

Hmmm, why? Previously you made a big thing about the recommended charging rate of 0.5 - 1.0C. Why now did you ignore that? Your batteries have a capacity of 2650 mAh, so a 0.5C charge rate would be 1.3 A. Your rate of 0.3 A is much too low for reliable termination.

The pack stopped at 13.52v. Input charge of 3412mAh. Temperature was around 100 degrees. I'm not sure if the termination signal was missed or not because the safety time limit was hit.

You probably have not done too much harm since 0.3 A is only a little greater than the safe overcharge rate of 0.1C (0.26 A). But remember what I told you. If you are going to do a slow charge up against the timer, keep the rate below 0.1C.

 

[poormanq45]

I took your information. You said for best leveling that you want to do a slow charge. I assume that is so that when the cells are nearly full charge none are overcharged too much, correct?

The recommended voltage per cell on discharge is .9v. Is that under load, or resting? Maybe that's where I went wrong.

*edit* All cells in all three packs are within 0.009v of each other when resting. That's a good thing, correct? They are all around 1.415v.

 

[poormanq45]

To address your statement about termination.

I tested the termination capability of this charger.

With a full pack that is at a settled voltage of 12.6v I applied a 0.1a charge. The voltage on the pack raised to 13.52v within 30 seconds. Then the voltage dropped to 13.2 and the amperage charged dropped to 0.0. The voltage returned to 13.52 and the charger stopped.

If I am understand the termination correctly, this is exactly what's supposed to happen, right?

It seems this charger is very sensitive to termination

 

[Mr Happy]

I took your information. You said for best leveling that you want to do a slow charge. I assume that is so that when the cells are nearly full charge none are overcharged too much, correct?

The recommended voltage per cell on discharge is .9v. Is that under load, or resting? Maybe that's where I went wrong.

*edit* All cells in all three packs are within 0.009v of each other when resting. That's a good thing, correct? They are all around 1.415v.

Yes, for leveling you want to do a slow charge, but the charge rate should not be more than 0.1C or one tenth of the capacity. Since your pack is 2650 mAh, 0.1C would be a charge rate of 265 mA. 0.3 A is slightly higher than that, so if the charger cannot do 0.25 A it might be better to select 0.2 A.

During the leveling charge some of the cells might be overcharged while the others catch up. Keeping the rate below 0.1C ensures the cells that get overcharged remain within safe charging limits.

Yes, the discharge stopping point should be 0.9 V per cell under load, or maybe 1.0 V in a pack with many cells. But the resting voltage after you remove the load should never be below 1.2 V. If the voltage goes below that you should charge them up again without delay.

There is no reason to routinely discharge a pack though. Reasons for doing so might include capacity measurement or running refresh cycles.

 

[poormanq45]

The reason I discharged the pack is that it's a "new" pack. I just bought the cells. I've read in alot of places that you should condition the cells. Is this true?

Thank you very much for the information about the dynamic vs static voltage. Hopefully I didn't damage anything too much.

 

[Mr Happy]

Yes, conditioning is a good thing to do. Conditioning consists of assembling the pack and then charging it at 0.1C for 16 hours. You do not need to discharge first.

After you have conditioned the pack you can recharge it after use at 0.5C or more (1.5 A, say) while allowing the charger to determine when to stop. You should repeat the conditioning process after every 10-20 normal charge cycles, or every 6 months or so, whichever comes first.

 

[poormanq45]

Excellent. Thank you very much for your help.