This site has some background info.
Charging is pretty innocuous.
"Lithium-ion degradation at varying discharge rates"
"It is found that after 400 cycles the cells discharged at
1 C and 3 C have capacities of 83.9 % and 81.4 % respectively
of the maximum capacity recorded."
That means the rate the battery discharges is slightly important.
Your battery is 36v x 4Ah
Discharging at 1C would be 4A or 144W.
Discharging at 3C would be 12A or 432W.
My line powered lawn mower advertises 1035W.
And would only draw that current in heavy grass.
Power drops when the motor is free-wheeling.
For your mower to match mine, it would discharge
at 7C (if the motor has the range to do that).
If the battery pack gets too hot, the safety
will likely cut it off.
And the controller in the mower also stops the
mower when the batteries get too low. That helps
prevent damage to the pack. You can't run them
completely flat - not allowed. The controller
will stop it.
There is a tradeoff in construction, between the discharge
rate allowed from the battery, and its total Ah rating.
You could limit the battery to discharging 1 amp, then
optimize the separator and plate construction for a
higher resistance. The end result would be the battery
could have a higher Ah rating. Maybe instead of 36V @ 4Ah
it could be 36V @ 8Ah. The disadvantage is, the current flow
rate available "won't cut the grass". This means not all the
battery types on the market, are good enough to cut the grass.
I found an article the other day, that compared various
battery products available, and their max-discharge-rate
versus their Ah-rate, and you can see the clever tradeoff
between packs with low internal resistance, versus packs
that have increased overall capacity.
And portable power tools should take some of these
things into account, when they pick a cell sub-type
for the job.
And that's not the limit on voltage. There is a snowblower
product with dual 56V packs. Only one pack used at a time,
like your lawn mower. The price of the packs would take
your breath away. Probably twice as expensive as the one
you got. You really have to wonder at the economics
after a while. And the snowblower in this case, has gone
from "completely unusable" as in previous years, to
"can now handle a light snowfall". There's still no battery
one that could hand 18" of snowfall. If you were in the
snow belt here, you'd have to buy a *lot* of spare packs
and the expensive pack chargers, so you could charge
packs in parallel on a snowy day. You would have to go
out once an hour, and remove the snow, so the buildup
would not exceed device capacity.
My next door neighbor likes these battery gadgets, and
he has to recharge his mower before he can do the back yard.
It takes two charges to do the whole property. He threw
away his previous battery mower after about four years
usage. He's great with gas (can repair small gas engines
like the breeze). His electrics just go into the skip
when the red LED comes on... Seems uninterested in fixing.
A guy in a pickup truck stopped, when he saw that mower
out for collection, and asked if he could have it. So
some people know these can be fixed. You would need
tabbed batteries and have a sense of adventure (in
case one catches fire because of a mistake you made).
Don't work in the house on stuff like that. Take it out
to the patio.
The packaging of the battery packs is normally made
in such a way as to discourage repair. That's the way
it was in the NiCd era.
How many cycles do you reckon you've put on it?
Are they likely to have been full-to-empty cycles, full-to-half, or what
kind of usage?
What do you reckon is the current capacity? 90%, 80%, 50%, 20%?
How hot does the battery get in use?
How old is it? How many winters has it been laid up?
I think there are things that could matter on life expectancy (and
therefore those sorts of stats could be relevant, looking at it at a
more granular level), like, if the mower was kept on maintenance
charge 24/7 and therefore the battery on 100% charge level, I believe
that alone can have an impact on capacity and/or life span.
So, I believe the idea storage charge is around 40% of capacity but
how would you judge that, unless there is some sort of charge state
indicator on the system?
This is the *biggest* issue with electric stuff like power tools /
mowers (not such an issue for those reasons with mobile phones that
may not be left on full charge for any length of time (we know it
doesn't do laptops any good)) and why some stick with petrol mowers as
they generally take less in the way of winterisation or require large
sums spent on them even if you don't use them (or especially if you
either don't use them or use them lots).
We have a battery vacuum cleaner (well, we will again, when the one
that daughter bought us, then borrowed, and has just replaced for us
is delivered) and because that doesn't need laying up during the
winter and is used reasonably regularly though a reasonable range of
charges, doesn't suffer from those issues as such.
Maybe a battery cleaner (or power tools etc) kept at a summer only
holiday home might need taking home and using sometimes to keep it at
Funnily, I pulled out an old battery drill that Dad used to keep down
their static caravan and I can remember it being flat every time it
was needed and quickly not lasting very long once charged. I've
noticed it's 7.2V so I may well pull the cells out and add a 2m cable
and with some Anderson powerpole connectors on the end that match all
my good quality 7.2V RC car packs and then use it as semi-cordless.
Cheers, T i m
It's all a bit handwaving, but charging 50% might count for half a cycle yes.
300-500 cycles is the spec-sheet number. I'm trying to work out whether the
OP is in that ballpark, or miles away (if they did say 40 cycles I wouldn't
expect it to be anywhere near losing capacity).
That's the tricky thing. If the charger was smart you could put it in
storage mode at the start of the winter - but then the charger would need a
resistor and cooling to dump excess energy and they don't want to pay for that.
I'm curious about this because I recently bought a used cordless vacuum. It
has an aftermarket battery in it, and when run in 'Max' mode the battery
gets quite warm, and then you put it on charge while hot. I'm wondering
whether battery heating is the reason people complain their vacuums need new
batteries, or whether they actually go through the rated cell cycle life of
This is one advantage of battery-sharing systems. You can take the
batteries out and use them in something else during the winter.
A similar thought went through my mind when I recently dumped a NiCd
cordless drill. Although a) it was knackered and b) motors have got a lot
better in the last 15 years. My £25 Aldi replacement is miles better in
Half cycle of the allowable DOD, depending on how the BMS is set. ;-)
As you say, unless someone keeps a pretty good log (or it's built into
the BMS) it's all a bit finger in the air.
Indeed. That's why I wondered if the mower / battery had any sort of
charge status indicator in which case you could keep mowing (or stop
mowing) when it shows roughly half full on the last mow of the season.
Good point. I have a Samsung Galaxy S7 that I had soon after they
first came out and have used every day since then. I normally put it
on charge in the evening (and take it up to bed in case of
emergencies) and might put it on charge again in the morning if I know
we will be out most of the day. Some years later it seems like it's
loosing capacity (it might run out by the end of the day) but
considering how many cycles it must have done, I think it's done
pretty well. Part of that might be that even though it supports smart
/ fast charging, I've kept it on a std / slow charger for the exact
reason you suggest, keeping the battery cool. I can also tell if the
GPS has been turned on in use but how warm it feels.
Agreed. My FatMax drill has two batteries and daughters Lidl drill,
jigsaw and circular saw have 3 between them so you can share the load
/ charge / cycles.
I've done it previously with a 'big' cordless drill that I bought
pretty cheap from the market (I know the guy and he rated the motors
and gearboxes etc). Being Nicad they have eventually failed so I've
emptied one battery (it came with two, both have failed for lack of
use) and I've just fitted a heavy duty twin core / sheathed cable with
some crock clips on the end. Not short of all sorts of 12V power
sources here so as ling as I'm not moving around too much, it's not
really an issue. ;-)
Well there is that, however, it's also nice to be able to leave bits
in various drills to cover the range of things you are doing (pilot,
clearance, countersink, driver).
Thinking of my RC car racing days ... even then some of the chargers
came with fans and cradles built in so the battery sat if fresh moving
air as it was being charged or cooling after a race.
Cheers, T i m
Those gauges are often based on battery voltage. It's easy to work out the
start and the end of the discharge curve, but the part in the middle is
mostly flat. If 80% is 3.78v, 50% is 3.72v and 20% is 3.68v (completely
made up numbers) it wouldn't take much for a voltage-based gauge to be way
Every tool I have that I've opened has two wires going to the battery gauge,
which means it's just a voltage measurement rather than a watt-counter.
(Possibly fancier Makita/etc do it better)
Mobile batteries are slightly different in that the charge/discharge
profiles are relatively small. You might have say C:h and the discharge
current might be C or C/2. If you have a drill or saw with C:h the
discharge current might be 5C or 10C. The battery works a lot harder and
gets hotter - in a phone generally the battery doesn't get hot of itself,
it's just by being next to hot things.
For this reason I bought two Aldi £25 drills :) It was cheaper to buy
another drill than a second battery. Back then they didn't have the
interchangeable battery system - and of course that means you have to buy
all your tools from Aldi...
(Should I mention the four Lidl pruning saws I bought for the same reason
I think some battery chemistries can deliver more current when warm - so you
win the race if your battery is fresh off the charger. I think that applies
to lithium ion too.
Agreed. However, I have some experience with the battery gauges on
disability scooters and the ones I've played with seem to have a
fairly representative charge indicator?
Start off on 5 bars, soon drop to 4, slowly go though 3, 2 and 1 and
then it stops. ;-)
Nope, I can assure you that I can tell when the GPS is on on my phone
by how warm it is. ;-)
Under normal circumstances (even when charging on the slow charge) it
feels cold to the touch.
Yeah, you often see that. Or get extra batteries 'included' for way
less than you would buy them for separately. The last time I did that
was with an RC Drone. ;-)
Hehe. Plus you have plenty of spares. ;-)
Sure, as long as they don't end up too warm etc.
I think most everyday battery suppliers / chemistries suggest an ideal
working temperature range. Lead acid (capacity) certainly suffer quite
a bit when cold.
Cars that won't start first thing in the morning but will later on
when it warms up a bit. ;-)
Cheers, T i m
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