Equalisation with a Difference

I think mixing Prius and Vectrix results is confusing to many.
I'm no expert on Vectrix, but I am one of the primary contributors on keeping older Prius running (on Prius Mk1 user group). I also keep my 14yo NiMH battery going in an 1998 Prius, for fun... though at this age, I am about to do a LiPo conversion (to my own plans) on the Prius.

Laird said "Certainly in the case of the Vectrix charge process 'overcharge' is the norm and it occurs every 12 or so hours of use and the damage can be seen in the failed batteries and/or the bulging cells which are contained within most (probably all) Vectrix batteries."

But regarding the Panasonic HHR650D NiMH used in the Mk1 Prius (NHW10), the Panasonic patent on the matter makes it clear that the long cycle life is achieved through 'shallow cycling' of the cells, and indeed the Prius controller keeps State of Charge (SOC) between 20% and 80%. Also, the HHR650D cells will deliver 300A on shorting, yet the Prius in practice draws only 55A upon electric use, and never charges above 45A (regenerative braking). So the current is kept under 8C, so within specs. I've read on this site that Vectrix draw 250A at top speed, which seems way over-spec, so no wonder cells fail so quickly in comparison.

The Panasonic patent regarding the HHR650D and 'vehicle use' of NiMH makes it clear that some rejuvenation of the NiMH cells is achieved by holding at 45-degC for 3-4 days.. but in practice, in car equalisation is achieved by periodically uniformly charging above 80% SOC, but still to 'knee' and then keeping low current running for a while. This is so that already-charged cells can easily lose that extra energy as heat, while the not-quite-full cells get that little bit more (slowly) to all end up equally charged (provided they are not already defective).

That same patent notes that the best periodic rejuvenation is achieved if you do three-deep-cycles of charge-discharge, using a computerised charger, set to NiMH mode. I've done this and the computerised charger (eg Bantamtek) does report larger storage (mAh) on the later cycles.

But as 'shallow cycles' is the secret to long life in NiMH, you don't want to do deep-cycling too frequently... and arguably deep discharge is one of the quick ways to kill NiMh cells. I still feel that slow/slight over-charging is better to discharge to achieve equalisation - and this is what the PEVE JV between Panasonic and Toyota decided to implement in Prius and have kept with for 15 years. So doing deep discharges to achieve equalisation and then using high-current draws in Vectrix, you might be employing BOTH known ways to quickly kill NiMH cells.
Graeme Harrison
prof at-symbol post.harvard.edu

Just a little thought.
I like the equalization idea.
It might be possible to add a fixed voltage at about 92V, then after a while all cells will end up with a voltage of 0,9V (+/- the tolerance of the resistor).
My personal experience after discharging Vectrix cells says it is only a few mAh left when voltage goes under 1,1V/cell, so a 0,9 discharge can be seen as completely empty cell. A benefit is that the built in charger can power up since Voltage is kept over 80V (I think 80V was the magic limit), and it might be more gentle to the cells.

Just a little thought.
I like the equalization idea.
It might be possible to add a fixed voltage at about 92V, then after a while all cells will end up with a voltage of 0,9V (+/- the tolerance of the resistor).
My personal experience after discharging Vectrix cells says it is only a few mAh left when voltage goes under 1,1V/cell, so a 0,9 discharge can be seen as completely empty cell. A benefit is that the built in charger can power up since Voltage is kept over 80V (I think 80V was the magic limit), and it might be more gentle to the cells.

What an interesting idea! Much easier than adding a diode at every cell level, it seems to fit in with The Lairds suggestion to use the KISS principle.

What would you use for the constant voltage source?

Could "Special Freddy" be modified for the purpose? The MR capacitor would need to be chosen so that Freddy produces 30mA at 92V. Or am I barking up the wrong tree here?

0.9V / 39.0 Ohm = 23mA; add to that the 7mA from the constant-on motor controller and you get a total requirement of around 30mA constant current to keep the battery at 91.8V.

Personally if I would go for a fixed Voltage, I would buy a "Lazy Frolle" :- ) from Ebay. Search on Ebay for "90V 300mA".
I have ordered two to put in series, to do a EQ-top-up-charge without heating.
One would keep the Voltage above 90V if the specifications are correct.

I hope it was not me who killed the thread. If it was, I will try to wake it up with this post. :-)
I did read http://www.camlight.com/techinfo/techtips.html
There it becomes clear that regular deep discharge is critical to achieve a long life of a NiMh-cell.

“Critical to proper maintenance of your NiCd and NiMH battery packs is an occasional slow charge to help balance the voltages of the cells and regular discharging down to 0.9V/cell to allow a full charge to break up any large crystals forming in the cells that can affect performance. It also breaks up any high-resistance compounds that might have formed on the electrodes due to passivation.”

“No matter what type of pack you have, bring the cells down to 0.9V/cell every month or so at a 1C rate or less. This full discharge allows a complete charge which breaks up any large crystals that might have formed in the cell. This increases the active surface area of the cell, lowers its internal resistance and increases the voltage under load of the cell. “

At the same time I did read above that Prius-cells never fails, and they are always kept over 20% charge. So I am a little bit confused.

At the moment it looks like I need to replace a few cells in my pack, and I think I will add a few resistors as The Laird did.
I think I will also add one of the 90V 300mA power supplies I have (inexpensively on Ebay) that will then level out all cells at just under 0,9V each. It would also be possible to add an external load of maximum 300mA to speed up the process of bringing the pack down to 90V. A diod is also recommended to put in series with the power supply so the higher voltage in the pack will not damage the supply.
1V and a 39 ohm resistor would give 1,25Ah drainage during 50h in the weekend, and 4,2Ah for a full week.
Adding a 60W 220V light bulb would add an extra 135mA of load to the existing 25mA.
Then a 50h week end can drain (135+25) x50h = 7,5Ah of discharge. An external load also increases the risk of reversing cells! But a 100-135mA current in the wrong direction are most likely not so destructive as the 185.000mA current the motor controller can pull from the battery. No, the fugueres above are not exact.

Keep in mind it is dangerous high voltage we are playing with!

Hi everyone,
I have read through this thread and the problem of unbalance in the NiMh VX1 battery has caused the early failure of many Vectrix's. The main cause of imbalance is uneven leakage currents in the cells causing an uneven charge throughout the battery. If the bike is used and charged infrequently then this becomes serious enough to damaged cells during run flat or full recharge situations. The uneven capacities of batteries throughout the pack will also contribute to the problem. I commend the Laird for thinking of a simple way to handle this problem. Once you have the theory worked out the best way is to try it.
For my thoughts I would try the resistor and diode shunt equaliser circuit. The diodes used would be 1 amp power diodes that can be purchased cheaply in bulk. The forward knee voltages would come within your 1% tolerance for the resistors. With a terminal discharge voltage of 0.7 volts per cell then thy are effectively flat. The advantages of doing this is that the pack would discharge to 60 - 70 volts and be easier to recharge. I also think that top equalisation is beneficial to the pack. With Vectrix this would be 1 - 1.5 amps for 2 - 6 hours.
When I test used Vectrix batteries I do a top equalisation and then leave the batteries for 2 weeks. The cells that are lower than the average after 2 weeks have high leakage and I don't put them in a replacement pack.

“Critical to proper maintenance of your NiCd and NiMH battery packs is an occasional slow charge to help balance the voltages of the cells and regular discharging down to 0.9V/cell to allow a full charge to break up any large crystals forming in the cells that can affect performance. It also breaks up any high-resistance compounds that might have formed on the electrodes due to passivation.”

When toy and other manufacturers started to use NiMH batteries along with the older NiCads, they tended to treat them the same - as if they were identical in characteristics. The crystals referred to occur in NiCad batteries, but not, AFAIK, in NiMH cells. Manufacturers mistakes like this can have unfortunate results; my EZIP SLA bicycle came with instructions saying to completely "cycle" the battery a few times to bring up capacity. Taking an SLA cell down to full or near full discharge WILL damage it. It will usually survive if new, but if you follow the instructions you are guaranteeing a short life for the battery pack. I ignored the bad instruction, but many conscientious people would follow it.

Good result, but can't see the logic?

I had pretty good performing battery pack and I was using the original charging firmware with care. Then I changed to the one modified by The Laird, and all my troubles began. But do not stop reading, the outcome is very good!

I got only about 8/17 of charge with the new firmware, and only about 22km of careful driving.
Well, I have 22km to my work, and I can charge there, so the bike was usable. But my impression was that a large portion of the capacity was lost…
I tried adding a 9Ah charge on top using 300mA current. But only got maybe 10-15% increased range, so no big difference. Things weren't looking so bright...

Then I came across this document mentioned above about let the cells go down very low from time to time, thinking I have nothing to lose. And I did not want the original battery-frying firmware back.
I did carefully discharge the pack down to about 12V, then charged it with 300mA for 20h (6Ah). Then I plugged the bike in, and I got 14/17 of the bars!!!
To good to be true I was thinking. But I was actually able to do a bit over 40km ride in pretty high pace. Do not know exactly when the last bar disappeared and I continued driving on the “hidden” 6Ah I did put in to the battery.

So at the moment, I am more convinced that letting the cells go low from time to time is healthy! And I was a bit frustrated over that The Lairds firmware failed to deliver on my bike, when everybody else seemed so happy. But now I am also happy! And I would be even happier if the result would be the same on my car with NiCd, but there it went from bad to worse (at the moment), but that is another story.
Resistors are ordered so soon I will also have a modified pack!

I will try the same procedure on a friends bike, also with a little too few bars and post the result.

I have followed this thread and seen several different solutions. From the single resistor to shunt regulator circuits adapted from Li-Ion battery designs. The previous post is about to modify a bike so I will explain the solution. The circuit I prefer that retains simplicity and gives effective equalisation of Ni-Mh pack with damaged high leakage cells is the resistor and diode. They are wired in series and then parallelled with each Ni-Mh battery.
The maths for this I will explain in an example. Battery 1 has 1.4v and is fully charged. Battery 2 has 1.2v and is partially discharged. When using a 39 ohm resistor the disharge current is: Battery 1 ( 1.4 / 39 )amps = 36ma. Battery 2 ( 1.2 / 39 ) amps = 31 ma. The balancing current is the difference of these and is 5ma.
When using a diode and 18 ohm resistor the discharge current is: Battery 1 ( 1.4-0.65 / 18 ) amps = 41ma. Battery 2 ( 1.2 - 0.65 / 18 ) amps = 31ma. The balance current is the difference of these and is 10ma.
The diode and resistor gives twice the balancing current of resistor alone.
There is another advantage of doing this. The battery will only discharge to 0.65 volts per cell or 66.3 volts for the pack. The battery would take a long time to reach this voltage as the discharge currents drop near the end. A practical statement is that the battery is balanced and at an end point of say 80 volts.
This bottom balancing would not have to be done every charge. It will depend on the pack and I would do this when I notice a drop in range. The balancers are still lightly balancing the pack all the time and the pack will take longer to go out of balance.
The diodes should be bought all from the one supplier, so that they are from the same batch.
I have found these on Ebay and suggest this.

http://www.ebay.com.au/itm/1N4001-SILICON-RECTIFIERS-DIODE-1-AMP-50-VOLTS-AXIAL-LEADS-Qty-100-NEW-/190965635695?pt=LH_DefaultDom...

http://www.ebay.com.au/itm/1-2W-Watt-18-Ohm-1-Axial-Metal-Film-Resistor-500-Pcs-/390503174929?pt=AU_B_I_Electrical_Test_Equipmen...

The diode must be wired the right way around. Once soldered I would heatshrink the completed balancer.
This modification when fitted to a rebuilt battery will extend its life by reducing damage caused by inbalance and overcharging or overdischarging during the life of the bike.
The best way to charge a Ni-Mh Vectrix battery is to charge it just before riding. This is not always practical to do.

Double spot, sorry

Hello The laird,Good idea but the use of the freddy charger is necessary.
I have the same idea for a Lifeypo battery that I want use for upgrade my Vx1. I want to use a 2,7 v Zener an a little resistance in series instead a bms for every battery. when I don't use my bike the zener diode go down the voltage to 2,7 volts and make the equalisation to down and after that, all battery have the same voltage before charging.

Best regards.