Re: [RoboMower] Removing ballast weights!
Thank you for the very comprehensive information, it would be nice to get away
from the sla batteries. I have found a 24 volt 20ah LifePo4 packs in my area, it
comes with a bms and charger for around $ 460.00. I would have no shipping
because I could pick it up. I wonder if I should take one for the team and see
how they hold up, or just hold on and buy another set of eb 20-12 b&b
From: Danny Miller <dannym@...>
Sent: Fri, April 8, 2011 12:35:40 AM
Subject: Re: [RoboMower] Removing ballast weights!
8 series cells of LiFePO4 works REALLY well. That maintains ~26v "fresh
battery" voltage for like 90% of its life, it does not slow down.
With the volumetric density of LiFePO4, 20AH 24v is around 1/2-2/3rds of
the volume of the batt case. But, the geometry of the cells will be
unlikely to match the available space well enough to fit more in. I had
20AH cells and there were 8 of them to make 24v. Can't just add 2 more,
the voltage would be wrong, all I could do would be to put 2 cells in
parallel for each one there now and there's nowhere near enough volume
for that, not with the geometry issues.
20AH is less than half the weight of 20AH in lead acid.
The problem is, there are difficulties in keeping the cells balanced,
and you cannot let any cell drop below 2.5v. Now you may say "well,
that's 20v and the mower will shut down before then." But the cells are
never totally matched for capacity, so you could have 7 cells at 3.0v
and 1 cell at 2.5v and that's 23.5v total, and the mower will NOT shut
off for that, and it will overdischarge and kill the lowest cell. So,
shutoff must always respect the lowest cell voltage.
There are many cheap BMS systems out there. A proper BMS will balance
the cells on charging AND provide low voltage shutoff. Many BMS do one
or the other and still call themselves a BMS, and a lot of them are
cheap and dangerously unreliable.
I've been considering engineering a really solid BMS, but right now I
just charge and balance them manually and have the mower set for its max
2 hr timed run which never runs the batts down anywhere near all the
way. But you have to be a skilled engineer with the right equipment to
do this reliably in the "manual" way.
When they're still in balance (seems to like 10-20 charges between
balancing), they're a dream to charge. I set the power supply at
27.4v. It's a auto-current-limited 3A max supply. It starts in a
current-limited mode with the voltage auto-reduced and it sucks 3A right
up until "full" and quickly drops to zero. There's no "tapering off" of
its charge acceptance as it reaches higher states of charge. It's VERY
efficient- the battery does not get warm on either charge or discharge.
Technically, the max "full" is 3.65v/cell, or 29.2v total. But there's
no reason to charge them that high because even 27.2v is already like
95% "full". After that, the voltage will rise quickly even though very
little charge is added, and there's a risk that a cell could overvolt.
At 27.4v, as long as the cells are basically in balance, no cell will
overvolt and they'll all get charged.
Manual balancing would be to put the supply on 3.55v and finish charging
each cell one by one to 3.55v and it stops taking current, and you move
the clips to the next one. If the pack was already charged "full", this
shouldn't take more than 5 min per cell.
I did see there's some 10AH-20AH LiFePO4 flat pouch cells which are
actually too LONG to fit in the battery case at all, in any orientation.
LiFePO4 batt market has been "not good" for awhile. All the quality
production is contractually committed to vehicle OEMs, the stuff that
makes it on off-the-shelf websites is often stuff that didn't pass QC or
was their lab samples that they torture-tested first. The eBay "duct
tape" bike packs are usually made of scads of small cells, which is just
risky because the failure of any ONE cell will probably trash the whole
pack for you.
Thundersky sold one "12V 20AH" that is actually in the EXACT SAME CASE
as the 12V 20AH "half-U1" lead acid batts. This has no BMS and no way
to install one, so it's doomed from the start to die a very early death.
But, there ARE more on the market every month, and the price is dropping.
On 4/1/2011 5:30 PM, Gordon Wojda wrote:
> Would you talk a little more about the batteries.
> I need new ones and have been looking at the LifePo4,
> 1. will they work,
> 2. what are the problems with retrofitting them.
> 3. I know they are costly to purchase.
> 4. That they have special charging requirments.
> you have experiance with them is it time to take a chance on them.
> thanks in advance.
> From: Danny Miller<dannym@...>
> To: RoboMower@yahoogroups.com
> Sent: Fri, April 1, 2011 4:44:06 PM
> Subject: [RoboMower] Removing ballast weights!
> Well, in addition to fixing the gearbox, I just took on a task I've
> wanted to try for a LONG time....
> Those huge steel ballast weights in front. They're heavy... clearly,
> someone thought that loading down the front sinks the front wheel into
> the grass so it doesn't "float" over it. But this strategy makes it
> much easier to lose traction the rear wheels, it increases the load on
> the battery, and it stresses the front wheel well which is already a
> critical flaw in the body's molding (had it fail once, it's quite
> unrepairable, ended up replacing the entire mower body, now it's showing
> white stress marks AGAIN). It also makes the mower quite heavy for
> carrying around, most of the mower's weight is those steel ballasts.
> Also, I have a special case- I switched to 20AH lithium-iron-phosphate
> batts, same voltage and all, but about half the weight. That reduces
> the weight on the rear wheels and does decrease the drive traction. The
> ballast weight, being where it is, does nothing to increase the traction
> from the rear wheels, and only increases the weight it has to push up hills.
> Taking them off is HARD. I thought the plugs inside were holding them
> in. Actually, they're pressed in over a tapered column, the plastic
> forms around them. I got them off with a heat gun, but lemme tell ya,
> it was REALLY hard, took over an hour, and I was seriously worried about
> ripping off the plastic stalks they sit on. Most of my fingers have 1st
> degree burns (no blisters but they sting).
> But yes, I got them off! Once I get the gearbox back together we'll see
> how it does. It might need some of the ballast back, but I'm certain
> the ballast already there went WAY overboard. They're 3.9 lbs apiece.
> The mower's sure a lot lighter now, anyhow.
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