Dual Battery Setup Help/ Ideas

Shakes

Finding my (electric) wheels
Jun 17, 2021
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2
Dear All,

My small foldable eBike currently has a very simple 36V 18650 10S2P 7Ah battery with an XT60 discharge and a 2 pin JST charge port. Within the bike casing, there is room for 2 batteries and I was considering what would be the best approach in order to install them potentially.

As for discharge, should I be looking to get simple parallel cables or battery discharge boxes? Unsure of the differences here.

As for charging, the bike has one universal? 9mm exterior charging port and I was really hoping to be able to retain this aspect in a way in which both batteries can be charged from one cable - perhaps a custom parallel JST connection internally? I understand this would be liable to current issues but I am unsure again.

Thank you for reading and any help is appreciated.
 

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matthewslack

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Anyone? Anything?
You can connect things together very simply, but to achieve a safe and foolproof setup needs proper thought and care.

For example, how to prevent a full battery being plugged into parallel with a flat one? This is a procedural issue not a physical connectors issue, and potentially rather dangerous. Really you need permanent connections, not easily pluggable ones so that the parallelling is done carefully once and then left alone.

There is also a need to be very careful with the battery on/off switches once connected in parallel. They must both always stay on, otherwise you have another route to accidentally connect batteries at substantially different voltages.

Charging two batteries with one charger: if they are in parallel, one will charge via the charge port, and the other via the parallel connection. But obviously quite slow.

All of this for generic systems only, not the likes of Shimano, Bosch etc..

Bottom line, if you don't fully understand anything to do with batteries, for safety reasons, don't do it. Use two interchangeable batteries instead.
 
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Nealh

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One has to be meticulous with how they treat such parallel systems.

Both batteries have to be charged separately as they have individual BMS, on can not charge them in parallel like RC lipos. Trying to charge In parallel with battery BMS will simply result in the first battery to reach a full balance simply switching the charge off and leaving the other battery at an unknown SOC.

Unless that black box has a blocking diode inside then the voltage of both batteries needs to be within 0.1 -0.2v of each other when connecting together as one, otherwise current will flow at a high rate between batteries and cause things to get very warm.

Discharging is fine in parallel as parallel doubles the ah/capacity and current rating.
As mentioned charging is a no with BMS controlled batteries.

We use to do lipo's in parallel via parallel board with 6 packs on the go at once but use a PSU to do so with a constant current spread between the packs, but they had no BMS and cells were monitored remotley via the jst connector for volatge.
 
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Benjahmin

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I have run two batteries in parallel for about 3 years. They are connected via a Y splitter made with Anderson Power pole connectors, red and black paired so no reverse connection is possible.
It gives me around 28Ah capacity and gives less voltage sag around the hills of Wales.
However one must be very disciplined.
ALWAYS charge the batteries seprately.
ALWAYS measure the voltage output of each battery BEFORE connecting them together. Mine read 41.6 - 41.7v each.
Although I charge with two seperate chargers I always finish both batteries (one after the other) with the same charger. This ensures an identical voltage on each, BUT I still measure before connection.

If one battery somehow becomes disconnected during a ride - DO NOT reconnect as voltages will be different. Better run down 1st battery, then disconnect and connect 2nd battery to finish your ride.
 

StuartsProjects

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Thinking aloud here, but if the batteries need to be charged seperatly, is there a problem in just having a switch that selects between the two batteries whn in cycle mode ?

The display would tell you that battery A is getting low, so you flip the switch to battery B.

And no, I have not tried it, if I want extended range, I carry the second battery in my rucsac.
 
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sjpt

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is there a problem in just having a switch that selects between the two batteries whn in cycle mode ?
You don't get the advantage of higher current capacity, less voltage sag that you get using both in parallel. I believe the Bosch dial battery setups switches between them (automatically).
 
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Nealh

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I think that is what that black box does, it doesn't use the batteries in unison. It simply connects both batteries then there is a switch inside, once the electronic detect battery A's voltage is low it switches to B.
One is best using the XT parallel leads and dealing with battery voltages as Ben & I have mentioned, one can't get slap dash with them.
 
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Shakes

Finding my (electric) wheels
Jun 17, 2021
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Here is a diagram of the black box, if it is of any help for those who may be interested in considering something similar - I think I will go for a Y splitter for discharge and build an additional charger port for the second battery to charge through.
 

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Nealh

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Yep as I thought cyphering the chinGlish on AliExp where I saw one, the MCU operates the Sw from batt A - B so they are not used in parallel a one.
If going the XT60 route one should integrate a Schotky to each battery unless one is rigourous with charging and connecting voltages that are close.