On using cordless tool batt packs for electric go-karts

[alim]

I've been picturing electric karts powered by Li-Ion cordless tool battery packs. Yes they're a bit puny, but the idea being that you would have several of them charged and ready at "home base" e.g. your garage, and then when a pack that is in use goes low, the rider just pops in for a pit stop i.e. grabs a fresh battery and plunks their dead batt on a free charger if available.

One of the things I've been scratching my head about with this idea is what voltage of battery pack would be suitable to what voltage of motor. What I mean by that is DC motors generally line up well with the voltages of SLAs i.e. 24v, 36v, 48v to match with the typical 6-cell 12v SLA batt x 1/2/3 in series. With Lithium packs, they don't seem to line up as well. I did a bit of research and here is what I came up with, forgive me if this is already common knowledge around here.

Typical Li-Ion cells are 3.6v nominal, 4v max. Considering only nominal voltages, batteries would then come in 3.6, 7.2, 10.8, 14.4, 18, 21.6, 25.2, 28.8, 32.4, 36, 39.6, 43.2, 46.8, 50.4, 54, and so on. The reason why we see "12v MAX", "20v MAX" tools and batt packs are really just marketing hype, a "12v MAX" tool is a 10.8v tool, and a "20v MAX" tool is an 18v tool.

So then looking at the above list of nominal voltages of available batts packs, they match up with DC motor voltages at 36v. So a 36v motor for the kart(s) and a handful of 36v or "40v MAX" batt packs would accomplish this.

Comments?

 

[anickode]

You could also consider running packs in parallel for added capacity. I know there are some cordless mowers that use 2 packs. I don't know if they are in parallel full time, or if they automatically switch between the two though.

You do need to match the drain rate to the battery. If you are depleting a battery in 5 minutes routinely, it will damage it.

Take cordless tools as your example. Drills, screwdrivers, etc, your pattery pack lifespan is pretty good... Maybe couple years of regular daily use. When you use those same battery packs in tools like reciprocating saws or angle grinders that drain them in mere minutes, the packs are worn out to the point of uselessness in a year or less. Or compare a 2ah pack on a high drain tool to a 4ah pack. Logic says the 2ah pack should last half the use time, but in reality they tend to be more like 1/4 to 1/3 of the runtime. That's why cordless angle grinders and saws usually come with the bigger battery pack, and drills and flashlights don't.

The correlation of drain rate to battery size is very important. An undersized battery will not only decrease your range, it will damage the battery from exceeding whatever it's rated drain rate is. Combine that with the increased number of charge cycles, and it just doesn't end well.

 

[Bbqjoe]

This is a fine thought, but really a dead end when using such small batteries.
I think your only real option would be the proverbial Flux capacitor.

 

[itsid]

Well that's rather simple..

each 12V SLA (AGM or simple lead acid battery) has six chambers.. every single one raises the battery voltage by ~2.3 Volts for a fully charged battery...
meaning your 12V battery has 13.8 Volts fully charged (some a bit less but 13.4 at the very least)

So instead of 2 SLAs you can just get one 25.2 LiPo pack and even a 28.8 pack wouldn't really harm...

More important is the other way around.. will the kart controller harm the battery?
A 12V battery should not drop below 10V in order to be rechargeable and with recovery in mind typically a 24V controller shuts down at 18-20V [undervoltage protection]

And while LiPos can recover from even lower voltages, IDK where that magical line appears tbh.
So 18V on a 25.2 pack will certainly be fine, but can you recharge a 28.8 pack if only 18V are left? (I can't tell)

Also, keep in mind that you only have so many recharges per pack.
So say you have five and need three per day I'd say after 6-8 months you will need to replace all five.
A bicycle battery is way more expensive than one tool battery.. but cheaper than five I'd say,
and holds a bit more charge and lasts at least as long.

'sid

 

[Bbqjoe]

I think the closest anyone will ever come to an efficient electric kart is the forklift.

 

[anickode]

Monster garage did build a 400 hp electric impala SS powered by something like 500 Milwaukee 28v tool batteries...

 

[Bbqjoe]

Monster garage did build a 400 hp electric impala SS powered by something like 500 Milwaukee 28v tool batteries...

Sounds like a Tim the toolman job!

 

[alim]

I think the closest anyone will ever come to an efficient electric kart is the forklift.

Electric karts aren't efficient?

---------- Post added at 08:48 PM ---------- Previous post was at 08:35 PM ----------

Well that's rather simple..

each 12V SLA (AGM or simple lead acid battery) has six chambers.. every single one raises the battery voltage by ~2.3 Volts for a fully charged battery...
meaning your 12V battery has 13.8 Volts fully charged (some a bit less but 13.4 at the very least)

So instead of 2 SLAs you can just get one 25.2 LiPo pack and even a 28.8 pack wouldn't really harm...

More important is the other way around.. will the kart controller harm the battery?
A 12V battery should not drop below 10V in order to be rechargeable and with recovery in mind typically a 24V controller shuts down at 18-20V [undervoltage protection]

And while LiPos can recover from even lower voltages, IDK where that magical line appears tbh.
So 18V on a 25.2 pack will certainly be fine, but can you recharge a 28.8 pack if only 18V are left? (I can't tell)

Also, keep in mind that you only have so many recharges per pack.
So say you have five and need three per day I'd say after 6-8 months you will need to replace all five.
A bicycle battery is way more expensive than one tool battery.. but cheaper than five I'd say,
and holds a bit more charge and lasts at least as long.

'sid

What did you mean by bicycle battery?

 

[anickode]

Electric karts aren't efficient?

From a practicality standpoint, they're so-so. A gas engine is cheap and will last decades and takes seconds to fuel up.

Battery packs have a comparatively short lifespan, high cost, and take time to charge.