Waterbottle ebike battery question


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3:55 PM
Aug 15, 2020
I want to make or buy a water bottle battery as an emergency backup for my Focus Bold. I know they sell the TEC add-ons but they are expensive and heavy. I just want something that might get me another 15 minutes. The question is, what is the most important thing I need to look for? Do I need to match Volts? I think the way the Focus add-on works is that it pulls totally from the spare when it is "plugged" in so whatever I come up with has to be able to stand alone in running the bike. Thanks
You need volts, capacity and C rating.

Volts to match the main battery
Capacity since that gives you range
C rating or multiple batteries in parallel to support how many amps the bike will pull.

I build my own packs but I started doing that when I did electric RC stuff. There is lots of information on batteries and the methods to build your own packs. Also you will want a BMS so you don't kill your pack.
To increase the capacity? Your problem there is going to be matching the existing individual capacity and chemistry to not create an unbalanced pack.

Depending on how the battery is surrounded can be a bit of an issue.
Maybe I can just make one from “scratch “. Any suggestions on a helpful website?
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The issue is the BMS needs to be configured to whatever Focus has for theirs which includes CanBus protocols that communicate with the controller and they are proprietary to the motor system, Shimano in your case, usually. It's not as simple as making a 36v battery with a pos/neg lead coming out of it.....If you can't find an aux battery for a Shimano system online other than the Focus one then there probably aren't any.
You are asking for problems unless you really know what you are doing.

Yeah, the tech pack is pricey at $600. Maybe a used one somewhere?

If you make your own, you'd have to use it like a TEC pack. That means no "addition" to your existing battery. You'd keep them separate. You might be able to set up a connector that will work if you can get the Lithium-ion pack to the correct voltage/amps.

Then, you have to think about charging it. Again, you have to get that right or you can risk a fire.

In the end, the emergency power you speak of is your legs. Shimano motors do not have much resistance unpowered. Yeah, it sucks, but we're talking an emergency here.

Ebike allow you to have a decent level of fitness. So, in a pinch, you might have to use that fitness and a bit more to get home.
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From scratch is easier but still you need to understand bottom/top balancing, capacities, C rating etc.... There are some good YouTube channels on batteries used for electric cars that explain those concepts.

What ever you do, do not solder the batteries get a spot welder or go with the connection kits. Heat destroys batteries and soldering Lipos does not go hand in hand.

This is far from a simple task and can cause fires if you don't know what you are doing. Much easier to just buy than build.

ziscwg mentioned the DIY lights that is a good safe place to start and learn since the voltages, amps and capacities are much lower.
What about replacing the current cells with the same amount of higher capacity cells, will that work? Obviously I am pretty battery ignorant, and should read up before I ask so many questions, but if I am headed down a rabit hole I can quit while I am ahead and either get a frame that uses the Shimano 504kWh battery, or get used to running out of juice at the 1 hour mark.

Here is a pic of the opened battery pack.

Building end to end like that is not the easiest thing to do with lithium ion. You can weld on tabs then fold to get a good connection.

Here's the thing the best batteries currently are Samsung which have around 3200mAh . Not cheap batteries. But the think about building your own pack is you are working with significant power in voltage and amps to cause harm and fires. Start small with something like a light battery to get the idea down on how to assemble and use a pack.

Jumping right to a e-bike pack I would suggest is not the best idea. You need to learn about voltages, C ratings, parallel capacity (amps), form factor, assemble techniques, bms etc.... It's not to learn but at high voltages and amps can be dangerous.
Here is a decent battery overview and about the safest way you could build a battery pack.

Still pretty high chance of a short which will cause a fire and noxious smoke.

The other option is look for an existing LiPo pack from the RC Car world. 3S has 11.1 volts and 4S has 14.8 volts, 6S has 22.2 volts. Putting batteries in series increases voltage additively but not amp hours (watts are amps x voltage, so a 10000 mah 6S battery at 22.2 volts would be 222 watt hours, a battery that costs around $120.00). Two 6S batteries in series would provide 44.4 volts at 222 watt hours. Putting batteries in parallel increases amp hours but not voltage. Two 222 watt hour 6S batteries in parallel would give 22.2 volts at 444 watt hours. Would take quite a bit of research to work out what would be needed for your motor and it very likely could damage your motor and likely void your warranty. So 4 6S 10000mah batteries at $120.00 each, 2 sets of 2 batteries in series that are then connected in parallel will give you 44.4 volts and 444 watt hours for $480 dollars as a comparison.

You can search LiPo packs under the "S" voltage you want, just remember if you overcharge or discharge to fast, they will catch on fire, explode, emit toxic fumes, so not to be monkeyed without doing the research, having the right tools for the job, and working in a safe area.

You would also need a good balanced charger will cost a couple of hundred bucks.

You would also need the right connector and a container/package to hold the batteries.

Here is what happens when Lipos go boom. Fast forward to 1 min 20 seconds...

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Good info!

Although I will disagree with the several hundred dollar charger if you put a BMS on the pack you can use a $35 charger.

But then I do build my packs top balanced for each "cell" using a charger from my electric RC plane days that does ipos so there is a few bucks in that.

shreddr as you can see this is not the easiest of undertakings. DIY is doable but you have to take your time and learn before you ever attempt.
The main reason I went with a couple hundred dollar charger is most of the cheaper ones can be difficult to use (chinese to english programming logic is wonky) and I have had 3 chargers that were under a 100 bucks go dead the first year of use and that was pretty casual use. This was for RC lipos with no BMS, just low voltage cut off at the speed controller. Reminds me I need to dust off a few cars and take them out for a run.