Parallel Second Battery Hookup


New Member
Ok, so I bought a 20ah ebike battery off Amazon which I put in my Topeak MTX bag. The Topeak bag is attached to the rear rack with the Topeak Omni QuickTrack Adapter. The battery fits in the bag like it was made for it. I just cut a slit in the corner of the bag for the wires to pass through. With this setup you can just unplug the battery and slide off the bag and take it with you if you have to leave the bike. I got the dual battery parallel connector from Bolton Ebikes and the thing works just like they say. I had to get some XT-90 pig tail connectors also from Amazon. You will have to open the the bike up and cut the wires to the controller which are the wires to the battery. The wires from the controller go to the male xt-90 on the battery connector and the batteries go to each of the female xt-90's. It is a pretty simple install because you are just connecting wires together. Now I haven't had a chance to take the bike for a long ride but I did go for a spin around the block and everything is working perfectly. It is a 20ah battery so now I should have 30ah and triple the range. I was only getting 17 miles so I was a little disappointed. Hopefully this will work as I think it should. Now if you don't now how to work with wires don't try this.



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Last edited:

George S.

Well-Known Member
I was looking at the Bolton stuff. It's clear that you can use any (check voltage, etc) battery you want with most ebikes if you tap the wires to the controller. The normal DIY battery has red and black output wires, and you match that to the controller. I don't access the controller. The first thing I noticed when I got my bike six weeks ago was that the pins for the connection between the battery and controller are fairly standard 1/4 inch spades. They aren't really standard because they are fat ( on the bike) so you have to take a spade and bend it out some. Not hard, but you don't want to ruin the tension. You can look at the battery and the connector on the bike to see how it works. The middle pins do nothing.

I removed the XP battery. It's a little too low capacity for higher speeds. It just slides out, so I changed nothing. But I put a spade on a black wire, and a spade on a red wire. The other ends of the black and red wire go to an XT60. No real need for the XT90. Ok, so slide the red and black wires, the spades, over the pins inside the battery case. BUT BE SURE you know which is plus and which is minus. I have one bike and I don't know what they do on all models. Anyway, once the connectors are on the fat pins, I put a strain relief in the wires and sent the wires out. I'm going to use the LFP packs I made from cells on Battery Hookup and Sriko. You need a spot welder. These packs are safe and long life.

Everything works. I don't understand the XP packs. The stupid key is in the way. It's not that easy to get the battery out. I can lift mine out. I love the integration, but it's just not ideal, the way they do it. I don't know why the lock is in the battery. It's my bike, so I guess I'll do what I want. I don't want to cut stuff if it voids the warranty, but the Bolton solution is more rigorous. Oh, well.

What you've done makes complete sense. Just buy a basic battery and use that. I don't really love using different batteries in parallel. I wouldn't leave them connected except while riding, and you want them at the same start voltage. Great project.



Well-Known Member
The Bolton adapter is a Y connector with diodes, so the batteries can be at different voltages, although I wouldn't recommend doing that. Diodes do fail eventually.

i connect my (same voltage packs) in parallel with a y-connector too. No diodes. Just have to be sober,


Well-Known Member
i connect my (same voltage packs) in parallel with a y-connector too. No diodes. Just have to be sober,
Same here. The Bolton product is fine but it is also limited to 30a draws and I am well past that :).

Paralleling packs together is something I have done for years but also something I caution people not to do if they have any alternative. There is a lot of chance for painful errors to occur. Which is also why I tell people to know exactly what they are doing with DIY battery packs before they attempt such a thing, and if they don't... don't do it.

2Fat is currently running two 16ah packs together making a 32ah pack. Each pack is identical in construction (cell count, cell type, pack design, same 60a continuous BMS) and the packs were joined on Day 1.

One thing: 2 packs in parallel may - on paper - double the BMS' capability, but in real life you do NOT want to go there. Two packs with a 30a BMS each are - joined in parallel - technically able to deliver 60a. Don't try to run a bike that needs 60a on that pack. Also a parallel'd pack that is say 14S4P x 2 is NOT going to handle sustained load anywhere near as well as a proper 14S8P pack will. The cell count is the same but the 8P pack will suffer voltage sag like an 8P pack whereas the two 4P packs will take a beating and sag like a hammock.


New Member


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Well-Known Member
Northeast Pennsylvania
Nice build and a clever way to carry your spare!

As Mr@Robertson suggests above, you need to use caution when wiring these lithium batteries. Is there a fuse or switch in the circuit? Connecting two batteries in parallel, that have different characteristic, can cause unexpected results unless the BMS is designed for this.

I usually just carry two batteries and swap them when necessary. Lately, the length of some of my rides exceeds the capacity of the two 52V 15AH batteries I use. Instead of carrying a third, I did what you did and bought a larger 52V 20AH battery. My bike uses a rear rack battery so I mounted the new one on the down tube, just the reverse of the way you did yours.


When I'm not using the larger battery, I insert this adapter in the battery mount. It lets me carry a water bottle and a small 4AH emergency power tool battery.


Even though both batteries are switched and fused, I installed this 30A circuit breaker as a battery isolator to prevent a parallel connection.


It's mounted under the seat so I can switch from one battery to another without stopping.