Spare battery just in case

[Bikes4two]

• I'm just settling in to my recently purchased TSDZ2 (36v 250w) and the 36v 10Ahr bottle battery
• On a not too hilly ride and with me putting in 'not a lot' of effort, I'm getting around a 50 mile range from the battery
• I'm now considering greater distances and maybe more challenging terrain, so I'm considering having a spare battery in my saddle bag, say maybe around 5Ahr, to connect up when the main battery is out of juice (and I don't want a bigger/heavier bottle or Hailong battery as for most of the time I'll not need it and why carry the extra weight around)
• I'm happy to use just a plug and lead arrangement for each battery rather than a 'smart' changeover/paralleling device, although that's not out of the question.
• A trawl of Ali Express brings up a lot of 36v 10S2P packs, especially for e-scooters as well as ebikes.

SOME TECH ADVICE PLEASE

• What characteristics/parameters should I be looking for in choosing a suitable pack?
• I'm guessing a BMS is essential but what BMS current should I be looking for - is 15A enough?
• And I guess I need to know something about the lithium cell's max discharge rate?

All tips/suggestions greatfully received - cheers, B4t

 

[Nealh]

15A continuous give little or no head room current wise, ideally 20 continuous.

Be careful connecting a range extender once the first is depleted as it will like try and reverse charge the depleted one via the discharge ports. Switching the battery off isn't enough one will physically need to remove the battery or fit an interrupter switch between the discharge wiring and the controller.

 

[harrys]

I have some small 36V4AH packs. I often run them in parallel with each other or with larger batteries. No protective circuits. Those are just Y-connectors with diodes anyway. I just use a Y-connector and am careful to check my batteries for full voltage before I connect them. I disconnect when charging.

The risk is someday I will forget and plug a depleted pack into a fully charged one. No doubt I will someday. Guess I should buy some diodes and make a protected Y-connector.