Best option for an auxilliary battery

[frank9755]

I'm looking into options for an auxilliary battery for my Wisper. The objective is to get a longer range.

My first thought was to add one or two NiMH 'boosters' along the lines of those that Nick/Tiberius has used to great effect on a Torq, as described on this post

However I'm a little bit wary of messing around with self-build packs. So I'm now thinking that the LiFePO4 packs that Jeremy and others have bought, and which seem reasonably priced, may be a simpler option. I could link this in via a schottky diode and get great range that way.

Does anyone have views on how long these packs are likely to last? Obviously we won't know the definitive answer for a year or two yet, but is there any reason to believe they will have better shelf life than Lithium ion batteries? Also, are they tolerant of deep discharge?

Any thoughts welcome.

Frank

 

[flecc]

They are still essentially lithium-ion batteries, mainly the cathode different, so will be likely to have the same preference for frequent charging. That's not necessarily important though, since they are apparently good for at least double the number of charges, that's a 1000, but in theory will last longer, to 2000 or even 4000 more charges, according to which prediction is chosen.

It follows that the life will probably be longer, but it's early days and these are the early entrants to the consumer market.
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[frank9755]

Thanks, Flecc - I'm quite tempted by the LiFePO4s. The fact is that you can't get an NiMH battery more cheaply and they are lighter. Also it would be just the job should I ever get my Tongxin working again...!

Frank

 

[flecc]

Yes, I think I'd try it in your position Frank. The only reservation I have is that manufacturer's like eZee and Wisper in consultation with their battery manufacturers are still avoiding adopting them yet, preferring to go with the latest standard Li-ion instead. They do still think of LiFePO4 as the next step though.

Why they don't adopt them yet I don't know, but I think the only possibility is failure to deliver current at a high enough rate. If so, you might find your auxilliary battery will need to be used at less than full throttle on hard climbs and the like.
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[Jeremy]

So far, my 36 volt 10Ah LiFePO4 seems to be able to deliver the maximum current that the Tongxin draws (about 15 amps, equivalent to around 560 watts into the controller) with a very small voltage drop (from about 39 volts off load, to about 37.5 volts on load). Although I haven't had a chance to do a full range test yet, it does look as if the battery voltage is very stable, which bodes well.

I would guess that one of these batteries would hold up very well for a fairly legal ebike. It's only on a bike that draws a lot more current than this that I think the battery might present a problem.

I have no idea what the life of these cells will be, but as they are reasonably cheap I'm not sure that this matters too much. If my pack died in a years time, I suspect that I may be able to buy a replacement for about half the price, the way prices are going.

Jeremy

 

[flecc]

I'm not sure of the Wisper motor power, but it's similar to the models used on some eZee bikes, both from Suzhou Bafang, and the eZee hits 20 amps and more at times. That's why I'm not too sure about the lithium iron capability with them. It would be good to find out though.
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[Beeping-Sleauty]

Suzhou Bafang

I'm not sure of the Wisper motor power, but it's similar to the models used on some eZee bikes, both from Suzhou Bafang, and the eZee hits 20 amps and more at times. That's why I'm not too sure about the lithium iron capability with them. It would be good to find out though.
.

may be able to answer that for you in a few days, hope to have one of these Wisper hubs up & running on LiFePo4 36v 10ah battery shortly, i have had no trouble with maximum current draw, not one cut-out, and the power stays constant, even on a 'thirsty' & very heavy bike,

 

[Wisper Bikes]

LiFePo4

We have been looking at these new Lithium batteries with great interest particularly considering their longevity, I thought that some of you may be interested in Lishen's comments?

"The latest generation of Lithium phosphate batteries are very interesting for a number of reasons :

1. They are very stable under abusive conditions (very safe)
2. They provide excellent cycle life (around 3 times better than existing systems)

They do however have some disadvantages :

1. The voltage is lower than conventional systems (although in your application this may not be a problem)
2. The energy density is lower than conventional systems (around 50%)
3. The technology is still new with limited production so the product cost is high.

I believe we are currently the largest manufacturer of Lithium Phosphate cells and we are producing for a number of well known battery brands so we have a good understanding of this product type. Lishen is currently promoting this product to replace the Nickel Manganese Cobalt type product you have previously used although I believe we will also introduce a Lithium Manganese system later this year which offers some of the safety benefits of Phosphate but with improved energy density and reduced cost.

I have attached some details of our current 11Ah model along with a brief presentation explaining a little more about Lithium Phosphate. If you are considering this as a new battery system for your bikes then I would be happy to help you in the development of such a product."

I am very interested to hear of your experiences with LiFePo4 especially concerning range.

I will upload Lishen's presentation, if anyone is interested.

Best regards David

 

[frank9755]

Interesting, thanks everyone.
One question I had about the LiFePO4 batteries which Jeremy or Beeps may have a view on is how hot they get. Would it be ok to use one inside a pannier, or is some sort of cooling needed?

Frank

 

[Jeremy]

I've not pushed mine too hard, but as far as I can tell it doesn't even get warm, either when discharging or charging. The BMS got very slightly warm during the initial charge/balance cycling, but even then I doubt if it was more then ten degrees above ambient.

I'd quite happily put them inside a pannier, as I don't think they present any appreciable risk from over heating.

Jeremy

 

[frank9755]

Thanks Jeremy,
I'll definitely get one then. Just need to decide if I need the 10 or 15Ah! The bigger battery is tempting but it is an extra 1.5 kg of weight. I'll mull it over for a day or two then decide...
Frank

 

[Beeping-Sleauty]

cool LiFePo4

Interesting, thanks everyone.
One question I had about the LiFePO4 batteries which Jeremy or Beeps may have a view on is how hot they get. Would it be ok to use one inside a pannier, or is some sort of cooling needed?

Frank

Whilst i was conditioning my LiFePo4, i charged & discharged it 'naked' that is, out of the intended case, even at continuous full load the battery didn't get warm, the BMS got a little warmer, maybe 10-12 degrees, should be fine in a pannier and doesn't need cooling.

if you have the space req'd, i would suggest the 15ah version, your range would increase and the extra weight is acceptable, my next project is to build a 'touring' battery, probably 20ah, to fit a Topeak rack bag so it can be demounted. with both batteries onboard i'm hoping for 60-70 miles range.

we shall see,
beeps

 

[frank9755]

Thanks Beeps,
That 15ah is appealing because it gives great range as a standalone should I get my Tongxin back in use for the summer.
As a battery to join up with my existing 13ah Wisper battery, it would give 70-80 miles, which is more than I can ever envisage wanting so, strictly speaking, the 10ah would be better for that.
The more I think about it, it seems more sensible to go for a battery that would have more additional uses, so I am leaning towards the 15Ah...!
Frank

 

[frank9755]

I've bought one of Mr Ping's 10Ah batteries. I decided that I only wanted it as an auxiliary for the Wisper, not as a battery in its own right, so 10Ah is more than enough. When I get it in parallel with the 13Ah Wisper one, I think the range will be adequate, even as both batteries start to deteriorate with age and lose some capacity.

What swung it was the weight saving of 1.5kg for the 10 vs 15Ah. The 10Ah, at 3.5kg, is the same weight as the Powacycle NiMH - which I've found a very comfortable weight to carry as a spare in a pannier.

It's being built and tested at the moment. I'll report back when I've tried it out.

Frank

 

[Tiberius]

Hi Frank,

(Sorry, I've been away for a few weeks)

What's the charging arrangements for the LiFePo4 battery? Is it like other Li batteries where the charger supplies power and the intelligence is in the battery pack?

One of the issues with multiple batteries is the chargers. I had both Li-Ion and NiMH and two chargers, but now the bike is completely NiMH. With two different types you need two different chargers, but with one type of battery you still need two chargers, unless you want to spend twice as long charging.

Nick

 

[Jeremy]

Hi Nick,

The Ping LiFePO4 usually comes with a charger included in the price. It's a simple affair, just a constant voltage supply with a current limit and current sense, like a lead acid charger. The charging intelligence is built in to the BMS and actively charge balances individual cells in the final stage of charge, by shunting current around each cell as it reaches it's maximum terminal voltage.

edited to add the following picture of the BMS board:

Jeremy

 

[Tiberius]

Hi Jeremy,

That's neat. I quite like the move to put the intelligence in the battery rather than the charger. It removes the danger of mismatching charger and battery type.

It would also solve all the problems we have been discussing with parallel charging. Just connect multiple smart batteries up to a single dumb power source and let them sort themselves out.

Now the electronics is a fraction of the cell cost, its clearly the way to go.

Nick

Edited to add:
It is perhaps worth remembering what the word "battery" means. It's passed into the language as an electrical power source but really it means an array or set. The electrical bits are cells, and only when there is more than one cell does it become a "battery". From there its a logical extension to the use of a Battery Management System.