Discharge Converters?

[AntonyC]

What I’m decrying is that these are on sale on aliexpress and ebay for small bucks up to big bucks, and we’ve got no simple means of knowing what the box is doing - whether they are safe, ineffectual, or otherwise.

Yeah, when I looked a few listings had FET-ty background images but all the descriptions would be met by any old diodes, so not much chance of a refund if they lose a volt. Best to ask sellers who only do bike bits and are well known, until you get a reply worth saving!

 

[Sturmey]

Sorry to nit-pick but did Ti draw those body diodes back to front??

No. The explanation is given below but note this is a slightly different/variation of the application that we want. Full document attached below.

 

[AntonyC]

Great info Sturmey, I'm glad you beat me to the Edit/Retract button.

Here is a top of the range battery blender at a top of the range price.

DATEx2 Parallel Battery Adapter - Big Game Bikes

Upgrade your eBike journey with the DATEx2 Parallel Battery Adapter – the 'battery blender' for seamless rides. Connect up to three batteries, handle diverse types, chemistries, and charge states. Say goodbye to inconvenient swaps! Voltage range: 24V to 84V.

biggamebikes.com

Website: "When you use a 52v battery and 48v battery then the 52v battery will be used first until its voltage is the same as the 48v battery at which point the batteries will run at the same time until the 52v reaches its low voltage cut off e.g. 42V."

I checked about charging and they replied as expected: "When charging with a single charger, you would be relying on the individual BMS of each battery to protect against overcharging. The DATEx2 doesn't manage the high voltage cut-off (HVC); that's the job of each battery's BMS."

Incidentally a 48V charger would charge the 52V battery in this scenario to around 75% SoC, still useful. Props to BGB for clear info.

 

[Cisco-man]

I’ve really enjoyed reading people’s comments and reading through all the technical notes - thanks. I think though that for my own requirements (a Swytch Max powered bike, but wanting to add additional generic batteries) I shall not go the reverse voltage protection route with mosfets. Instead I’ll make it impossible to reverse connect the batteries by using good connector practice. I’ve ordered up some 30A schottky diodes to stop potential reverse feeding of the disparate batteries and I think for me that will be enough. I just need to test rig it and see what sort of diode temperatures I get under various loads so I can mitigate for any chance of thermal runaway. My hope is that I can house the diodes in the Swytch controller case but if not then there is space on the back of the case for a little diode “home”.

 

[Sturmey]

I’ve really enjoyed reading people’s comments and reading through all the technical notes - thanks. I think though that for my own requirements (a Swytch Max powered bike, but wanting to add additional generic batteries) I shall not go the reverse voltage protection route with mosfets. Instead I’ll make it impossible to reverse connect the batteries by using good connector practice. I’ve ordered up some 30A schottky diodes to stop potential reverse feeding of the disparate batteries and I think for me that will be enough. I just need to test rig it and see what sort of diode temperatures I get under various loads so I can mitigate for any chance of thermal runaway. My hope is that I can house the diodes in the Swytch controller case but if not then there is space on the back of the case for a little diode “home”.

Here is one of my versions. Its a 40 (20+20) Amp 60V dual (common cathode) mounted on a piece of aluminium.