Current State of the Art in Battery Balancers?

This is exactly what the Prolong grid charger does. The Prolong uses a low 0.35A current to charge the entire battery pack. When I use it to charge my battery pack for 20 hours, the voltage peaks at 244V (1.45238V per cell).

 

Thanks for this info. I was thinking grid chargers would be likely to apply charging currents in this range (100mA up to 500mA).

In my own testing I've used "low" charging currents as low as 5mA and as high as 500mA when doing that final bit of charging which "balances" all of the cells in the pack to identical per-cell voltages.

I call this a "top-off charge."

I apply the top-off charge after finishing a normal charging cycle with -dV/dt, cell temperature, total mAh charged, a simple timer, or some other method for ending the charging cycle. And I wait until the battery pack cools down to room temperature before starting the top-off charge. I use a timer to prevent overcharging. A top-off charge of 100mA adds 100mAh to the battery every hour. 10 hours of top-off charge adds 1,000mAh (1Ah) to the total charge which is already inside the battery pack. For 6.5Ah Prius batteries it might be better to end the top-off charge after 5 hours, adding only 0.5Ah to the existing 6.5Ah.

I recently discovered that a top-off charging rate of 100mA is sufficient to achieve the "fully charged" level in a long string of cells. I have done this test with individual cells to get the voltage from each individual cell.

After a pack receives 100mA for 10 hours, the per-cell voltages converge into a very tight range and remain incredibly stable, changing by only a few mV during several hours of data recording. I'm able to measure voltages with 6.5 digits of precision. While this isn't needed for ordinary charging, it has been useful in order for me to confirm that per-cell voltages for "fully charged" NiMH cells fall into a very small range.

The exact voltage is nearly always between 1.4000 and 1.4200V.

Curiously this appears to be the same for many different ages, name brands, and physical sizes and shapes of NiMH cells. However, cells of the "low self-discharge" or "LSD" cells fall outside of this range. But the cells in the Prius aren't low self-discharge. Note: Low self-discharge cells hold a full charge for several years. They are intended to replace alkaline cells in low-power applications, but permit being recharged when needed instead of discarding them

Based on recent testing I classify "per-cell" voltages as follows:
(These are measured with 100mA of charging current applied during the measurement)

• 1.4000V: This is essentially "fully charged" but not overcharged
• 1.4100 to 1.4300V: This is a bit more than "fully charged" but not overcharged
• 1.4500 to 1.4900V: This gets into the "overcharged" category. Do not keep a cell this high for more than 10 hours. Also monitor its temperature at all times. It should not rise more than 5 degrees F above room temperature.
• 1.500V and up: Overcharging is occurring. This could damage a cell if it gets too hot. Do not exceed the manufacturers maximum rated temperature. It has been said that lengthy overcharging, even at low currents, may reduce the mAh capacity of a cell. Continuous long-term "trickle charging" is not recommended for NiMH cells.

 

Based on my recent DIY grid charger result on my Gen 3 traction battery, I found:

1. It could not reach your "overcharged" territory. The max was 1.433V (240.7V pack) on the first balance. During subsequent balances, it never reached 1.42V (238.6V pack).
2. It could not stay in the max voltage for 10 hours. Typically within a few hours or shorter, the pack voltage will drop 1-2 V.
3. I suspect temperature has some effect on the balance (or top-off) voltage, if you want a per cell voltage precision of 5 and scale of 4.

 

This is reassuring. It indicates the manufacturer of your grid charger fully understands the details of charging and balancing a Prius hybrid battery.

I get higher cell voltages (during charge) when the cells are cool. I'm going to put some cells into the refrigerator and then measure their voltages during charge when starting at 40 degrees F rather than the current 76 degrees F in my workshop. I wonder if a portion of the -dV//dt voltage drop which occurs during charging is caused by the cells warming up rapidly after they obtain a full charge?

 

Looking forward to seeing your results.