Has anyone considered adding a LifePo4 "booster" battery?

And it would reduce safety by at least 95% !!!

And it would reduce safety by at least 95% !!!

ROTFL. I love to see these well reasoned responses.

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Harry - it sounds like a good idea to me. Some things to consider:

1) You'll need a good BMS for the LiFePO4 pack.
2) You'll need to consider max amp draw of the system (likely documented elsewhere on this site). With such a parallel setup of different chemistries I'm not sure how one would classify the draw on the LiFePO4 pack. If it were an additional NiMH pack then I believe you just sum the Ah's - however, with different chemistries I'm not so sure that's valid. With a small 10Ah cell you might exceed the maximum recommended discharge current. Whether that would end in apocalyptic melt down or simply degrade the life of the pack I'm not sure.

I'd say it's worth a well controlled experiment. In other words don't throw it together and forget about it. Monitor it after each use. Check for high temperatures. Don't charge it unattended until you've seen it behave properly a few times. Etc. Common sense stuff when dealing with something new. All this should reduce Mik's 95% by 93.457% (I can show you the calculations if you like) ;-)

You may find that it works well - or you may find that it's not reliable. However, personally, I wouldn't dismiss it out of hand.

Hi Harry,

I like your idea BUT it is not practical.

In battery technology, putting any batteries in parallel is generally 'frowned upon'. If one battery does something 'interesting' (like a cell going short circuit) then the other will suffer.

Mixing different 'chemistrys' is also very much a no-no. There will be different characteristics for charge and discharge and these will not mix well.

If the LiFePo have a lower internal resistance then the liFePo will discharge at a higher rate than the NiMH and visa versa.

The whole idea of mixing batteries of different chemistries and/or characteristics is potentially dangerous.

DON'T DO IT.

The Laird

But isn't that how the Vectrix fuel cell was going to be attached? http://car.pege.org/2006-eicma-vectrix/fuel-cell.htm

If it is just a battery charging a battery instead of a fuel cell it should work just like all those iPod/iPhone battery chargers that are just batteries themselves. No?

-Randy

There will be different characteristics for charge and discharge and these will not mix well.

The whole idea of mixing batteries of different chemistries and/or characteristics is potentially dangerous.

My impression was that Harry will be charging them separately. If the BMS on the LFP pack also monitors for both over voltage and low voltage on a per cell basis then I would have thought that there shouldn't be much risk of "bad stuff happening" to the LFP's.

Don't the dangers you speak of usually occur because of over charge or under charge conditions occurring? If we prevent those on the LFP side is that enough or would it also require the same on the NiMH side?

BMS is another matter. The few systems that can do >60V are very expensive and will cost twice as much as the batteries.

Check out the Goodrum / Fechter design available from tppacks.com - it's reasonable cost and they can be chained together.

Hello again folks,

My comments were on the reasonable understanding that these batteries would be connected in parallel ( plus a diode for charging separation). That is what HarryS said in the original post.

The biggest danger (to the batteries) would be that the battery with the highest voltage and/or lowest internal resistance would be supplying the most current and there is no way to change this easily.

As one battery ran down (out of charge) the other would pick up the load. If one battery was well down when the load was removed then the other would probably try to charge it until they were both equal in voltage. If either one went short circuit the other would definitely discharge into it and that could be explosive.

There are too many things to go wrong and not enough benefits to warrant/offset the risk.

As for the fuel cell. I think that you will find that the fuel cell would be acting more as a battery charger on the fly, as it were. This is quite a different arrangement. The charger would stop (by design one would hope) if the load became excessive (short circuit battery). If the charger (fuel cell) stopped charging there would be no particular problem other than a loss of power.

If the LiFePo battery was used to power a D.C. to D.C. converter and the controlled output of the converter used for charging the NiMH battery 'on the fly', then it could become a practical arrangement.

I trust that the extra weight just under the passengers backside has already been considered. It might make the bike a little less stable with the higher centre of gravity that would result PLUS it would restrict the passenger load i.e. total all up weight.

Only trying to be helpful:-)

The Laird

Only trying to be helpful:-)
The Laird

You are being helpful - I was looking for clarification. Thanks.

I think a DC-DC converter capable of enough output power would likely be impractical. But I'm sure the Laird is much better positioned to consider that.

If such a converter isn't practical then I now understand that this would likely be a bad idea because even small imbalances between pack A and pack B could potentially cause large current to flow because of low resistance of the packs. (At least that's what I'm understanding from what the Laird has written).

Thanks for taking the time to explain.

And the cables between the two packs would have to be thick, unprotected by resistors or diodes, and run along parts that could get exposed by accidents.

The seat storage area is designed for a carton of beer or other harmless (??) things. It is not sturdy enough to hold a lethal voltage device which cannot be shut off in case of accident.

And then there would be those electrocutions caused by roadside attempts to find/fix faults.

Batteries of this voltage must be contained properly, or people will die.

And the cables between the two packs would have to be thick, unprotected by resistors or diodes, and run along parts that could get exposed by accidents.

The seat storage area is designed for a carton of beer or other harmless (??) things. It is not sturdy enough to hold a lethal voltage device which cannot be shut off in case of accident.

And then there would be those electrocutions caused by roadside attempts to find/fix faults.

Batteries of this voltage must be contained properly, or people will die.

And you should never run with scissors.