How unsafe are custom batteries?

[skyeg3]

Hey guys this may not have a straightforward answer but it is on my mind a lot and its making me uncomfortable. I have a custom go kart I put together a few years ago. I had EV3MV (not around anymore) put together some nice 20s5p 18650batteries with BMSs. We charge this thing with cheap Chinese chargers that say WARNING all over them about fire risk. We charge these batteries in a garage with no sheetrock on the interior next to a gas car. EV3MV was a professional organization but I'm still not satisfied about the safety level given that the batteries do ride around in a vehicle that shakes and goes over bumps often. So, what do I do? How can I mitigate the fire risk? Are there good chargers I can buy? is there a way to test the batteries? Should I charge them outside? Should we throw a fire blanket over them during charging? Are there any very high quality battery BMSs I can buy that I can assemble into a new pack so I can sleep at night? Is there anyone on the planet that can assemble a very safe custom battery? Or is that an oxymoron? I know the pictures aren't great but its what I can find on my phone.

 

[Comrade]

Is there anyone on the planet that can assemble a very safe custom battery?

You can have a battery with a lot of safety features, and you can have a safe battery. Those are two different things.

I prefer the latter, meaning I use a commercially available battery made for power tools and lawn equipment, by a brand that sold tens of millions of units of these batteries. And if there is anything wrong with them, statistically I'm much more likely to read about a recall in a newspaper, than experience the fault myself.

My guarantee that my battery is not going up in flames is that there are millions of users out there with the same design that are not having issues. That's a safe battery.

 

[E-HP]

I had EV3MV (not around anymore) put together some nice 20s5p 18650batteries with BMSs.

Just making sure that EV3MV isn't the same as EM3EV, which is still around.

Battery Pack Brand Official Store | Em3ev

Em3ev Official Store offers batteries for E-surfboard, E-Foils, E-Motorcycles, E-Bikes and more online, Contact Customer Service for Coupons.

em3ev.com

I don't charge unattended, but you could put the battery in a bbq grill or surrounded by cinder blocks, but you won't avoid smoke damage. I would charge outside if you have an area for it.

 

[lnanek]

You can have a battery with a lot of safety features, and you can have a safe battery. Those are two different things.

I prefer the latter, meaning I use a commercially available battery made for power tools and lawn equipment, by a brand that sold tens of millions of units of these batteries. And if there is anything wrong with them, statistically I'm much more likely to read about a recall in a newspaper, than experience the fault myself.

My guarantee that my battery is not going up in flames is that there are millions of users out there with the same design that are not having issues. That's a safe battery.

Too bad the capacities are so low on tool and lawn packs, though. The highest capacity Greenworks 60V battery is 8Ah, for example. It takes me 20Ah to commute to work. So I'd need to bring 3 and stop to swap along the way, or have a lot of extra dodgy wiring and mounts to use 3 in parallel. Instead I just used a single 22Ah eBike battery and charge it in a metal ammo case with venting added on a concrete floor garage with nothing flammable nearby.

 

[Comrade]

So I'd need to bring 3 and stop to swap along the way

The Ryobi batteries appear to have a diode on the output. So running them in parallel should be easier than it seems.

 

[nicobie]

Don't charge them inside and unintended no matter what chargers you use. I'll never trust a BMS to save me from an 'incidence' (tool batteries being the exception). A cheap fire blanket from Amazon will help keep the flames down so as to not start anything close by from catching fire. Maybe you could buy a metal shed to keep the cart in while charging?

 

[electric_nz]

If they are brand name 18650 they have inbuilt safety mechanisms inside each cell.

If a cell dies the overall battery capacity will be noticeably reduced, that is your red flag that the battery has problems.

It has a BMS which will likely prevent overcharge and over discharge.

If you’re worried about the chargers check with a meter that they are the correct voltage.

There is very little difference between a well made custom battery like that and a factory made one. It will likely fade away as the cells age.

 

[skyeg3]

@Comrade that is a great school of thought. The closest thing I think I could find would be a Sur Ron battery.

@E-HP Yes thats them. Good to see they are still around. Although I don't believe they are making custom packs anymore. The BBQ grill is a good idea. Or outside.

@Inanek Ammo case, good idea.

@nicobie Fire blanket is probably easiest. Metal shed would require a new piece of property.

@electric_nz They are Samsung 25R cells. So by noticeably reduced what do you mean? 20%? Because I'm not entirely sure I would notice 20% actually. I don't have any good data to use. The chargers don't show a voltage. The only voltage read I have is from the cycle analyst. The batteries are in parallel so I wouldn't notice if one was low. I suppose I could start unplugging one battery at a time and looking at the voltage of each at the end of rides. But if a cell goes bad does that actually mean that the voltage goes down? These are 20s5p so I believe I'd see a reduction in current, not volts. The controller is the current bottleneck now, not the batteries I believe. So I may not notice a performance decrease.

Thanks everyone!

 

[Comrade]

There is very little difference between a well made custom battery like that and a factory made one.

Factory made batteries are made by robots with very expensive QC at every step. A safe battery is more about consistent techniques than a "safe" design.

Here is a Ryobi BMS. This PCB is at least a generation more advanced and better made than any after-market BMS I saw.

 

[lnanek]

There are so few of those FETs, though, and they are so small:



Last ebike battery I took apart had much bigger ones, and more of them in parallel:



We'd have to check the data sheets to be sure, but that might be the difference between something like supporting 13A continuous discharge without overheating for a power tool vs. 50A for an entry level Grin Baserunner ebike controller.

 

[Comrade]

It's only a 3 parallel cell pack. 13-15A is about right.

 

[nicobie]

We'd have to check the data sheets to be sure, but that might be the difference between something like supporting 13A continuous discharge without overheating for a power tool vs. 50A for an entry level Grin Baserunner ebike controller.

True Running a ebike for an hour or two is different that running a tool for 10 sec. at a time

 

[Comrade]

is different that running a tool for 10 sec. at a time

The lawn mowers these batteries were designed for run at about 400-500 watts continuous until the battery depletes. Certainily not enough for the speed demons, but more than plenty for a regular e-bike that might need 100-200 watts of assistance.

 

[lnanek]

That picture looks like a Ryobi 18V. Product page says 20 minutes run time with a 4Ah battery:

RYOBI ONE+ HP 18V Brushless 16 in. Cordless Battery Walk Behind Push Lawn Mower with (2) 4.0 Ah Batteries and (1) Charger P1190 - The Home Depot

Update your outdoor space with this RYOBI hd 20 ONE HP LithiumIon Cordless Battery Walk Behind Push Lawn Mower w Two Batteries Charger Included.

www.homedepot.com

4Ah consumed in 20 minutes is 12A current. 18V * 12 = 216 watts. My ebike uses the cheapest, weakest controller Grin Technologies sells, and runs 48V * 30A = 1,440 watts. It's definitely not a speed demon. Tops out around 28-32MPH. Still uses 7x what that tool pack is spec'ed for.

 

[docw009]

I have run a 36V ebike with a 20A controllers off Ryobi packs 2AH-5AH, I was OK with the 2AH model, but the 5Ah must have had a BMS that didn't like 10A and higher. It shut off. People did better with the EGO brand.

Scooter packs made by Segway, Ninebot, and other big names seem to make safe batteries. Almost always, the nickel strips on the cells are directly soldered to the BMS, which extends the length of the battery, You aren't going to see balance wires falling off, or getting crushed by the packaging, THere are several temperature sensors in the packs to catch overheating,

I admit I haven't looked inside recent packs made for Bosch and other expensive ebikes.

 

[calab]

The smaller tool pack batteries have less parallel groups making them less powerful discharging amps.
Tje sa,e company with 5s1p brick vs 5s2p brick might use different cells inside. You could use the tool brick batteries without the bms, install your own bms or do it visually with rc voltage lcd's or 6-8s voltage display. The good news is you can easily solder, quickly without adding to much heat, solder the tabs easy, real easy! Make a 10s brick ready to go, no lego sets, like with lipo even too.