Nimh batterys can be put in Parallel ??

[pgt400]

My buddy works for Toyota Service, has a Prius pack for me. Came off an 04' with 33k miles. Front end totaled. I think he said it was a 220v 7 AH pack? Can Nimi's be used in parallel? I remember from my NIcad/NIMI RC days (now all lipo) this was a no-no. Was thinking of a 60v 24 AH pack. Maybe use high amp diodes from each string?

 

[ZapPat]

I think its better to put the cells in paralell groups, then each group in series, but I am not sure if there is a big difference if done the other way... maybe someone with good NiMh knowledge could confirm?

Anyhow, it would be good if you had a cell analyser to test and match cell capacities and look for duds/weaklings too.

I have a large number of NiMh packs here that I want to use, and they are 8.4V/4Ah (7cells) per pack. I would really like to use them as is without having to dismantle them, but I am also wondering what setup would be best for this situation?

One thing is for sure: Avoiding complete discharge of the pack would avoid damaging the weakest cell(s), and from what I've been reading on this forum, incomplete recharging might also avoid killing cells by overcharging. I think this is why these hybrid car packs last so long in the first place. Of course this means a little less pack capacity too...

 

[johnrobholmes]

You can run them in parallel, just heed the advice of ZapPat for a long lived pack.

 

[fechter]

I don't know about that.

My experience is to run Nimh or NiCd batteries is parallel, you need to have separate series strings. Each string must be charged separately, but can be combined for discharge.

The problem is when one cell reaches full charge, the voltage drops. If you have cells in parallel, the first one to reach full charge drops voltage and hogs all the charging current resulting in thermal runaway. On cell can draw current from it's parallel neighbor when this happens, making the runaway worse. I've blown up a few NiCds learning this the hard way.

You could possibly charge them in parallel at a very low rate (<0.1C) where heating is not an issue.

 

[dirty_d]

i bought 150 cells already to make a 30s5p pack, what im going to do is have a 6 terminal connector off the pack, all the positives to one connector, then all the negative terminals get their own pin, the corresponding connector on the controller will put all the negative pins in parallel, on the charger they will be separate however, so each string gets charged with its own constant current. from what i hear using diodes to isolate each string during discharge is unnecessary and possibly harmful. im going out today to get the parts for the charger, it will be a simple C/10 15 hour charger. if you want a higher rate of charge then you will either need to make a switching regulator to convert 120V to whatever you need at the charging current, or a big expensive transformer. ill post the finished schematic after i build it and make sure it works.

 

[johnrobholmes]

I don't know about that.

My experience is to run Nimh or NiCd batteries is parallel, you need to have separate series strings. Each string must be charged separately, but can be combined for discharge.

The problem is when one cell reaches full charge, the voltage drops. If you have cells in parallel, the first one to reach full charge drops voltage and hogs all the charging current resulting in thermal runaway. On cell can draw current from it's parallel neighbor when this happens, making the runaway worse. I've blown up a few NiCds learning this the hard way.

You could possibly charge them in parallel at a very low rate (<0.1C) where heating is not an issue.

Good point there. I have always ran mine in series strings, with series banks in parallel. Then each pack was charged separate to avoid overcharging.

 

[ZapPat]

I don't know about that.

My experience is to run Nimh or NiCd batteries is parallel, you need to have separate series strings. Each string must be charged separately, but can be combined for discharge.

The problem is when one cell reaches full charge, the voltage drops. If you have cells in parallel, the first one to reach full charge drops voltage and hogs all the charging current resulting in thermal runaway. On cell can draw current from it's parallel neighbor when this happens, making the runaway worse. I've blown up a few NiCds learning this the hard way.

You could possibly charge them in parallel at a very low rate (<0.1C) where heating is not an issue.

Good point there. I have always ran mine in series strings, with series banks in parallel. Then each pack was charged separate to avoid overcharging.

Hum... I didn't think of that aspect of NiMh's charging, very good point indeed!

Ok then, so how many cells can be in each string so that we can charge at a fairly good rate (maybe 0.5C or so) without risking damage to the cells? I know it gets hard to detect the dV/dt of any one cell in a string the more cells there are. I suppose the temperature (dT/dt) of any one cell would be easier to detect if all the cells in a string are physicly close to one-another, but is only practical up to a certain point I would imagine?

What setups do people use when dealing with long strings of NiMh cells? How to prevent under and over-charging? I've got a few kWh worth of packs here I really want to use! I welcome any practical suggestions / ideas that people use to solve these problems!

 

[Jeff]

My buddy works for Toyota Service, has a Prius pack for me. Came off an 04' with 33k miles. Front end totaled. I think he said it was a 220v 7 AH pack? Can Nimi's be used in parallel? I remember from my NIcad/NIMI RC days (now all lipo) this was a no-no. Was thinking of a 60v 24 AH pack. Maybe use high amp diodes from each string?

Do not connect multiple NiMH cells in parallel. Isolated strings of individual cells can be connected in parallel through diode isolation, although it is a waste of energy.

There are a number of reasons why this is unsafe, and has probably been accurately discussed here before.

Jeff

 

[fechter]

What setups do people use when dealing with long strings of NiMh cells? How to prevent under and over-charging? I've got a few kWh worth of packs here I really want to use! I welcome any practical suggestions / ideas that people use to solve these problems!

I don't think there is any real limit to how long you can make the string, look at the Prius or Honda. They monitor the voltage and temperature of sub-packs in the long string. A microcontroller does a bunch of math on it to detect the battery state.

Of course I can't even program a Basic Stamp, so that approach is out.

Others have used multiple temperature sensors along the string so the charging current would be dropped as soon as any one gets too hot. This seems like a good approach as long as the charging current is reasonable.
I think you can just charge indefinitely with no controls below around 0.1C.

 

[ZapPat]

What setups do people use when dealing with long strings of NiMh cells? How to prevent under and over-charging? I've got a few kWh worth of packs here I really want to use! I welcome any practical suggestions / ideas that people use to solve these problems!

I don't think there is any real limit to how long you can make the string, look at the Prius or Honda. They monitor the voltage and temperature of sub-packs in the long string. A microcontroller does a bunch of math on it to detect the battery state.

Of course I can't even program a Basic Stamp, so that approach is out.

Others have used multiple temperature sensors along the string so the charging current would be dropped as soon as any one gets too hot. This seems like a good approach as long as the charging current is reasonable.
I think you can just charge indefinitely with no controls below around 0.1C.

Ok, this makes sense on how these are monitored. However, it makes for a fairly complex system when one is using small cells that require multiple long strings to be monitored. This makes me understand why knuckles likes the klixon solution!
Knucles - how many cells do you monitor with one single temp switch (klixon type)? Doesn't each cell have to be very close to the switch/sensor for the system to be reliable? The cells in a string being monitored at a single point would have to be physicaly close to this monitoring point for this system to work well (specially at faster charge rates). I can imagine small groups of cells hudled around the sensing point would work well, but larger strings of 40 or so cells in series would seem to require multiple sensing points to be reliable. And of course this implies that as soon as one switch opens in the string, it cuts the whole string and not only the group being monitored. This means cells in an individual string have to be fairly well matched for capacity (Ah's), right? If not, the "balancing" part of the charge where the switches oscillate slowly on and off might take quite a while, no?

A request for the people who have bought old NiMH hybrid prius packs: Could you photograph the internals of the factory packs to give us an idea of how toyota does assembles their packs? As much detailed info as you could give would be great!

I'm thinking of using a PIC with multiplexed temp sensors (or just use the available 10-12 ADC inputs on the bigger PICs) for each string to get a similar result as Knuckles gets with his klixons, but we could now set any temp cutoff we want, have any hysterisis we might need, and be able to reduce charge current of the string as each group of cells heat up. This takes initialy longer to make than using simple klixons and may not be for everyone, but once one person writes the program, other can use this same program for their own packs, making this solution potentialy interesting. Is this idea too far fetched?