18438Re: [RoboMower] Lithium time!
- Feb 10, 2013On 2/10/2013 11:01 AM, The CZ Unit wrote:
> Well, it's time to start building the packs for my Robomowers. I'mHi Christopher,
> thinking Lithium, 10-15ah capacity at 24 volts. Over the winter I have
> converted my Roombas and Scooba to 18650 type cells, and they work *great*.
> The BMS I am using is simple monitoring, it's up on Ebay, up to 8ah load
> capability with per cell monitoring. Since the Robomower doesn't pull
> that much current, a pair of 4 cell monitors should do it.
> Anyone with thoughts?
It seems like a good idea. It's something I should do myself! I actually
have a pair of Enersys/Hawker "Genesis" AGMs in my Robomower now, and
they're likely to last for years. But lithium plays right into people's
"I want it" mentality. So, it's something I need to get experience with,
if I ever expect to sell anything to anyone.
The trouble is, stuff for lithium sells no matter how badly done it is.
:-/ They make big promises, it fails in a year, and the seller sneaks
off into the night with your money.
Is this a project you're just doing for yourself, or something you want
to sell to others?
If it's to sell to others, is this going to be a cheap "make a fast
buck" product? Or something that costs more and is supposed to last?
Most of the 18650 cells I've tested are only good for 1-3 years of life.
Is that OK? They seem to be the best option for a quick-n-dirty project.
They also burn ferociously if misused. How much consideration do you
want to give to safety?
The A123 cells last a lot longer, but also cost more. We could build a
really nice battery pack with them that could last 5-10 years.
Frankly, I think the Robomower would be better done with a few large
cells rather than many small ones. This greatly simplifies the charger
and BMS design. In the extreme, it is perfectly practical to use *one*
3.2v 100ah cell with a 3v-to-24v boost converter. This makes the charger
and BMS issues essentially disappear!
An engineer can do for a nickel what any damn fool can do for a dollar.
-- Henry Ford
Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm
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