Re: [RoboMower] Lithium time!
- Is that English?
On 10 Feb 2013, at 17:01, "The CZ Unit" <cz@...<mailto:cz@...>> wrote:
Well, it's time to start building the packs for my Robomowers. I'm
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?
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- That'd be my concern, working with li-manganese 18650 cells is pretty
dangerous in larger packs. The chance of a single cell failing goes up
proportionately with a great number of cells. Normally, with a
separator failure in a single cell, the most heat it can generate is the
energy from what's in that cell alone. But with a macrocell of
paralleled cells, it'll absorb the energy of all of them, and fire is
more likely. A thermal runaway where one cell ignites its neighbors and
inevitably the whole pack is also likely.
Once you build a pack out of >50 cells of cheap li-mn, the chance of
catastrophic, intense fire is just too likely.
On 2/10/2013 1:26 PM, Lee Hart wrote:
> 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
- For the Roombas (14.4 volt packs) I went with eight Sony 18650 type
cells with 3.0ah capacity. These came out of some older Sony laptop
battery packs; "dead" usually means the batteries just drifted a bit.
However for the Robomower I would go with larger size cells, I was
thinking Headways at first, a 10ah cell would work pretty well, 8
needed. Or I could go with 10ah-15ah prismatics, not sure what would be
I like Lee's thought of using one big battery and a DC-DC, but the
problem there is the primary side would need to handle something like
80-100 amps to provide 10a on the output side. Does there exist a DC-DC
that could do this?
- On 2/10/2013 7:22 PM, The CZ Unit wrote:
> For the Roombas (14.4 volt packs) I went with eight Sony 18650 typeMore often, I find one of the cells (or parallel pairs of cells) has
> cells with 3.0ah capacity. These came out of some older Sony laptop
> battery packs; "dead" usually means the batteries just drifted a bit.
gone bad. The rest may still be good if you catch them soon enough
(before they go dead too, just from sitting).
> I like Lee's thought of using one big battery and a DC-DC, but the100 amp MOSFETs are only a couple dollars, and there are dozens of
> problem there is the primary side would need to handle something like
> 80-100 amps to provide 10a on the output side. Does there exist a DC-DC
> that could do this?
switchmode controller ICs that could be used. The main parts that
require careful selection are the MOSFET, output diode, and inductor.
If I were designing it, I'd build it as two twin converters, running out
of phase so one was on while the other was off. This minimizes input and
Also, since I know the boost ratio, the inductor would be tapped to act
as an autotransformer. This keeps the duty cycle on the MOSFETs and
diodes around 50% even though you're boosting about 8:1.
If we knew what we were doing, it wouldn't be called research, would it?
-- Albert Einstein
Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm