Hybrid battery balancing

AFAIK, the criterion is about temperature rise. It ought to be possible to balance at 5A as long as all the modules stay at the same, reasonably low temperature. What is reasonable? As a guess, probably mid 30'sC. You might possibly need some sort of fan cooling, depending on ambient.

 

There is an advantage to discharging at higher currents in that it saves time. The problem arises in monitoring that discharge to get an accurate capacity measurement on each discharge cycle. So that any improvement in capacity can measured and recorded.

John (Britprius)

 

At 5 amps as nh70 said the temperature rise will be greater than at 700 ma, but since the batteries are designed for 100 amp discharge I doubt this will make much difference. Temperature can be a factor though.
A discharge of 5 amps will exercise the modules just as well as 700 ma, but module capacity will appear to be less as the discharge rate is increased.
The important thing is they should all be measured for capacity at the same rate of discharge to get meaningful figures for comparison. It should also be pointed out that a discharge rate of only 5 amps will mean the load will give off around 40 watts of heat.

John (Britprius)

 

How low V/cell are people discharging the Prius packs? Going deep and chancing polarity reversal has much less risk of cell damage when done at low current loads. Too high of a load, and the cell voltage collapses once all of the active material is used up. If the voltage collapses too fast no current flows, no work is done and nothing is actually happening. This is seen when load is removed and the battery voltage rebounds dramatically.

 

The modules are discharged to around 1 volt per cell, 6 volts in total.

John (Britprius)

 

Set the discharge to cut off at 6V. That's 1 V per cell.

 

Great info as usual John, so if your charger does anything up to 1A discharge, is there any real advantage to discharging at the lower rate of 0.7A?

Would the readings be more reliable the lower the discharge current is set? Variance?

 

If anything the lower the discharge rate the less the figures mean in the real operating environment since the HV battery would normally be seeing much higher charge and discharge rates in normal use.
The lower the discharge rate the higher the capacity of the modules will appear to be due to the effect of "Peukerts Law".
This does not mean that reasonable capacity balance figures cannot be reached at low rates of discharge, but these capacities may not be reached in operational use. The Prius only uses 40% of the capacity of the battery in normal use.

John (Britprius)

 

I learn more by the day, thank you!

So if I was using a temp sensor on my battery charger setup, what would you suggest to be the maximum battery temp limit?

 

Maximum battery temperature should be set at 45 degrees C. I should point out that if you are going to charge the modules out of the battery pack they should be clamped in a vice or with "G" clamps with substantial timber supports on either side to prevent swelling, and if external cooling can be arranged "electric fan" this will help to keep the temperature down along with internal pressure.
If charging in the battery pack with a multiple chargers, do not charge adjacent modules and run the cooling fan with a 12 volt supply to keep things cool. The greatest temperature rise is when individual cells within a module reach full charge and the energy supplied is wasted as heat. This is required to allow lower state of charge cells in that module to continue charging and reach the same level of charge. Thus balancing the charge in the cells in that module.

John (Britprius)

 

Thanks again John. I have some spare modules I am going to experiment with first before I tackle the full battery pack. I will clamp them in a vice between two pieces of wood when charging and it is winter now (still way in the minus temps) so cooling is not an issue at all. Will definitely do alternate modules when charging the entire pack though.

 

I agree that the capacity readings will be meaningless at lower discharge currents, but for recovering/improving the actual performance of the cells, lower current=better.

The higher the load, the less 'work' will be done by the cell. At higher load, the cell voltage collapses once all of the active material is used up. Once the voltage collapses, no current flows, no work is done and nothing is actually happening. This is why when you remove the load, the battery voltage rebounds dramatically. Lower currents allow the cells to support the load for a longer time period, thus maximizing the mAh output and inactive material reconversion.

We have done extensive testing of this for our forthcoming pack level advanced deep discharger (that will work with both Toyota and Honda battery packs). 1V/cell is considered deep discharge to us and high current discharging at that level significantly increases the risk of cell damage during polarity reversal. The advanced discharger intelligently varies the load in order to maximize mAh output and inactive material reconversion while minimizing the potential for cell damage during polarity reversal.

 

Great insight Jeff, thank you!