Thoughts on batteries

[Jeremy]

I've just bought myself an electric conversion kit for my mountain bike, which has caused me to do some research into modern (but not state-of-the-art) rechargeable battery technology. It seems that by far the best bang-for-the-buck comes from high discharge 18650-cell li-on cells, which are safe and user-friendly things (compared to spontaneously-exploding li-po cells for instance) and entirely suitable for informed DIY hobbyists to use to make up into custom packs etc.

But more to the point, 18650 cells are the type most usually found in laptop batteries and cordless tools and so on, meaning that they can be obtained really quite cheaply - there are lots of Ebay sellers for instance selling new-but-obsolete laptop batteries for pennies on the pound, and lots of videos on Youtube showing people building such cells into battery packs for all sorts of purposes.

The amazing thing I've learnt though is that the huge battery packs in the Tesla EV are also constructed of exactly this same type of cell, and Tesla are now moving into selling even bigger battery packs again using this same cell for off-grid home use - ie., direct replacements for the usual large arrays of lead-acid batteries. Which of course immediately made me wonder about the merits of using 1860 cells to build a much superior (much more storage, much less weight and size) house battery bank for a bus. Might be time to start systematically buying cheap laptop batteries on Ebay...

Jeremy

[Iceni John]

Technomadia are using lithium batteries (I don't know which type)  for their bus's house bank  -  they've written all about their trials and tribulations on their blog.   Most folk however feel that Lithium batteries are not yet cost-effective for use in a bus, where weight and space limitations are not as critical as in an aircraft, for example.   Good ol' FLA batteries such as golf-cart GC2s still offer the most bang for the buck.   They're certainly not perfect or even ideal, but they're still viable for use in a typical bus conversion.

Maybe in another five or ten years we'll see lithium approaching the overall cost-effectiveness of FLA, but until then they'll probably be used only by a few technically-savvy and adventurous folk willing to try relatively new technologies.   Heck, plenty of us here are still using ancient 2-stroke engines, so that shows how we stand for being on the cutting edge of white-hot innovation!

John

[Lostranger]

Hey Jeremy,

Thanks for posting your research. I had no idea about the versatility of the cells you mention, but I intend to go to lithium ion batteries when our 8D AGMs need to be replaced. If it's good enough for Tesla, I'm interested.

We full time, and our bus is mostly off grid with 1020 watts of solar. We do not use a generator, but I'm rebuilding a ThermoKing APU that will recharge battery banks while producing heat or A/C for the back half of the bus. Better/lighter batteries would be a wonderful thing.

I'm also interested in knowing more about your bicycle conversion. We do not pull a toad, and I do much of our resupply travel on a good, touring bicycle. We also have a tandem, and I'm thinking about electrifying it. Keep us posted.

Jim

[Lostranger]

Been doing some research. Exciting possibilities for the off-grid community.

Jim

[Jeremy]

I'm also interested in knowing more about your bicycle conversion. We do not pull a toad, and I do much of our resupply travel on a good, touring bicycle. We also have a tandem, and I'm thinking about electrifying it. Keep us posted.

Jim

I've gone for the Xiongda motor which is the first one on the market to have a 2-speed gearbox built into it; it will either change gear automatically depending on your speed or you can choose the gear by flicking a switch. The clever thing though is that to change gear the motor simply stops and changes direction - you get forward motion whether the motor is running forwards or backwards, but at two different gear ratios. Which is neat. Only problem I've got at the moment is that my mountain bike has hydraulic disc brakes, but they've sent me the version of the motor for rim brakes....sorting that out now.

Jeremy

[RoyJ]

The flashlight community (Candlepower Forums) probably has more battery enthusiasts than just about anywhere. High powered lights puts a huge amount of drain in relatively few cells, and often push them to and beyond their electrical and thermal boundaries.

It's been a few years since I've been deeply involved, but from my limited knowledge, I'd consider two things crucial: lithium iron phosphate chemistry, and built-in cell controller. LiFePO4 allows both high drain and ensures a non volatile reaction if things go bad. The built in cell governor prevents both over-discharge and over-charge, together making the cell fool proof.

With old laptop batteries, I'm not sure if either of those criteria are met. Easiest way to tell is nominal voltage - if it's 3.7V per cell like most laptop packs, then it's not LiFePO4 (3.2V). Most laptops are low drain devices that do not benefit from the higher drain (but lower energy density) ability of LiFePO4. Plus, laptops have very sophisticated charging algorithms that makes in-cell protectors redundant.

Finally, to get the capacity we need at 1.5 Ah per cell 3.2V, we need 4 x 300 = 1200 cells for a 12.8V 450 Ah pack! I love soldering but even I'd get tired of soldering that many cells! Not to mention it'll cost you a pound in solder...

Commercial packs may be the only way to go at this point, but few of us are willing to experiment with a $3000 pack plus all the associated charge controllers. Maybe if you're full timing, but at 5 trips a year, I definitely can't justify it myself.

[Lostranger]

Thanks, Roy. Certainly helpful info.

Our house system is 24v, and I'm presently using 4 8D AGMs. They're one year old and in excellent shape. I'm making preparations to add a much smaller 12v system to start the APU and run a few 12v loads. This system will have a single, dedicated panel and will also be charged by the APU or by shore power.

I was planning to buy a fifth 8D AGM for the 12v system, but I'm now thinking of purchasing a few hundred new 18650 cells from China and seeing what I can come up with. It will cost less than one AGM and will certainly be lighter.

Any thoughts?

Jim