Can Ni-MH strings be equalized by shallow discharges, gradually increasing???

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Mik
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Can Ni-MH strings be equalized by shallow discharges, gradually increasing???

In the gargantuan XM-3500 forum TANWare suggested to equalize the Li-Ion batteries like this:

With this in mind it is best then with this type of ballance algorythm to break in the battery pack. While you would do this at your own risk an acceptable methodology, considering 40 mile real world, would be to make three 4 mile runs charging between each (10%). Then two 10 mile runs (25%) and then two 20 mile runs (50%). Lastly at least two no more than 30 mile runs with charges inbetween (75%). Remember it is important to use the included charger to fully charge the pack between each run.

This breakin period should, at least theoretcially, force an equalibrium between the cells on their capacity charge levels. I would especially not trust the pack is fully charged out of the box with all cells to the same capacity level. And if you take it out and let the voltage cutoff kick in without proper cell levels you are certainly placing some cells at dire risk.

Does anyone else have experience with this approach and is this likely useful for Ni-MH cells?

The battery which I would like to use it on is of course the 102-Ni-MH-cells-in-series-battery in my Vectux.

I guess it cannot do too much harm and I'll get started trying it out!

Mr. Mik

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Re: Can Ni-MH strings be equalized by shallow discharges, gradua

that process only works for batteries that have significantly greater shunt current loss at higher states of charge.
It most certainly wont work for thundersky lfp40s.
If you added a 3.6v zenner and a 1 ohm resistor to each cell it would.
though it wont hurt to do it either, just wouldnt help.

im actually quite interested in how xtreme got their chager to detect when a single cell hits 4.25v and shuts down.
this means the BMS required is really really simple. (ie a zener and resistor for each cell, no monitoring since the charger stops charging when one cell reaches 4.25v).
its the monitoring and automated charger shut down thats difficult (expensive) to implement, especially when dealing with high voltage packs (288v+)

from what i understand, nimh has very significant shunt current loss when charging to 100%.
so it *might* work.
however it also has significant shunt current loss when charging at a low state of charge, so dont go far.

Matt

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah
Vectrix 60Ah Lithium Tyres Fuel Registration Insurance cycle analyst 2 x TC Charger & MC
conversion

Mik
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Re: Can Ni-MH strings be equalized by shallow discharges, gradua

however it also has significant shunt current loss when charging at a low state of charge, so dont go far.

Matt

Thanks, I'm trying to understand how those Zener diodes work....

If I understand this correctly so far, then there might be more of a BMS hidden between the Vectrix battery cells than meets the eye...

These diodes and resistors could be in there without an obvious cable-boom leaving the battery.

Anyway, what do you mean with "so dont go far." ???

Mr. Mik

This information may be used entirely at your own risk.

There is always a way if there is no other way!

Mik
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Re: Can Ni-MH strings be equalized by shallow discharges, gradua

In order not to 'hijack' the XM-3500 thread I quote TANWare's reply here:

Since NiMh is memmory free as well the break in procedure, for a non BMS monitored array, would not hurt the cells. This could be a mixed camp though as I've heard others say to fully range a cell for the first five charges to break it in as this then gets the best overall capacity out of them. While the 5 cycle method has never been proven, some swear by it.

Thanks!

I did many more than 5 deep discharges initially, with poor results.
Even including running the battery down with the wheel spinning freely.

But I have never repeatedly done short rides interspersed with full recharges.

This is because where I live the real world trips (= the trips that get me to a place I actually want to go to), are always at least 20km long through 80km/h roller-coaster terrain before I get (back) to a power point.

Unless I ride purely for the purpose of this battery equalization attempt, of course.

I have started to do this procedure, but it will take some time in order to apply plenty of additional cooling between cycles.

I want to start each recharge with a battery temperature of 20°C or close to it, to avoid experimenting my battery into early retirement...

Mr. Mik

This information may be used entirely at your own risk.

There is always a way if there is no other way!

antiscab
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Re: Can Ni-MH strings be equalized by shallow discharges, gradua

zener diodes appear as open circuit until the voltage across them exceeds their break down voltage, at which point they go short circuit.
combine with a resisor, and you have a shunt load that comes on above a certain voltage.

its a very simple circuit, and could possibly have been included in the manufacturing process, but i don't know if it has.
for nimh, i doubt it has, there wouldnt be any point, the chemistry has a signifcant enough shunt loss at high state of charge while undercharge as it is.

by dont go so far, i mean, dont go far enough that the batteries will suffer from shunt loss while they are charging at a low state of charge.
from memory this happens at 50% dod or below, but dont quote me on that.
for something the price of a vetrix, it most certainly should include a BMS so you dont *have* to worry about any of this.
since ive never seen a vectrix apart, nor own one, i dont know whether it does or not.

Matt

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah
Vectrix 60Ah Lithium Tyres Fuel Registration Insurance cycle analyst 2 x TC Charger & MC
conversion

antiscab
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Re: Can Ni-MH strings be equalized by shallow discharges, gradua

whether deep cycling a non BMS monitored nimh pack is bad depends upon what happens when you try to further discharge a completely dead cell.

without a BMS, you can be guaranteed the cells wont be particularly balanced, especially if they are brand new and just commisioned.

again, for something the price of a vectrix, you would expect this to have been sorted out before delivery.

Matt

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah
Vectrix 60Ah Lithium Tyres Fuel Registration Insurance cycle analyst 2 x TC Charger & MC
conversion

integra 2.2t
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Re: Can Ni-MH strings be equalized by shallow discharges, gradua

i dont know if this will help but it kind of sounds like when i would equlize my cell for my r/c car

i use to race for years 7.2v and 8.4v r/c cars and battery performance played a big part (7.2v 2800mah and run time of 5-6 min)

i had the 6 cells (sub c 2800 mah 1.2v )laying side by side and wired to 7.2v

i used to break them by running them from full charge to when the car dies about 10 times

after that i would charge the pack and place it into a rack of restors that discharged each cells at 10 amps. while they where discharging i would keep record of the cells voltage and time untill the cell would reach .5v

after that i would plot a graph of each cell and see how the pack is doing

i would take the best cells (highest voltage over time) and place them together for my best packs

i also had a charger that charged the packs at 10a untill peak voltage and had a 20a boost for the start of the race

hope it helps P.

()_()
(o.0)
(> <) .... never trust a bunny !!!

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