Two batteries, one motor

[Yak]

So, I tried asking about how I could get a secondary battery to act with the internal battery of my second hand Orbea to work to extend the crappy range, and the guys at ebike batteries suggested two relays and a three way switch, 1, off, 2. Has anyone done this? It sounds supremely simple and effective, so that the batteries can work independently. I’d love to see a wiring diagram or any thoughts of the Pedelecs gurus…

 

[Nealh]

Personally I would parallel making sure the voltages pretty much matched (within .0.02v before connecting together. Not sure if the Orbea does any handshaking, but I would extend the discharge wiring out of the battery base/controller and in line on each battery feed place a shottky diode arrangement and then connect up the range extender. The shottky's will prevent one battery charging the other and both batteries should deplete voltage wise almost evenly, the battery with a higher capacity will supply a bit more then the other one if they differ.

As long as one is strict with their charge routine no issues should emerge, it will be better then an arrangement of switches.
Two batteries acting as one will stress the cells less as they all will be supplying half their normal current.

 

[Yak]

Thanks Nealh, much appreciated-but may I clarify a couple of things?
1. Will it be difficult to get batteries to within 0.02v if they are different ages?
2. If the Shottky’s prevent one battery discharging into the other, do the batteries need to be so closely aligned?
Lastly, the battery guys said:
“Using blocking diodes would result in 0.7V voltage drop with a silicon device. This is a huge amount when dealing with lithium-ion batteries when the
fully charged state of the cell bank is 4.20V and fully discharged at around 3.0V. Point seven of a volt is a big deal then.”
3. But aren’t we talking about 1 shottky (actually 0.3-0.4v drop from what I’ve read) per 36v battery? So the voltage drop isn’t a big deal?

 

[Yak]

Surprise surprise - my Googling of shottky diodes has lead me back to the Pedelecs forum where this has already been discussed. Sorry Nealh. Normally I search here first. There’s even a How To. Brilliant forum this.

 

[Nealh]

Although the schottky has a power surge loading, it is still good practice to have battery voltage fairly much aligned.
How much voltage drop one see's will depend on unit temp and current drawn.

 

[Andy-Mat]

So, I tried asking about how I could get a secondary battery to act with the internal battery of my second hand Orbea to work to extend the crappy range, and the guys at ebike batteries suggested two relays and a three way switch, 1, off, 2. Has anyone done this? It sounds supremely simple and effective, so that the batteries can work independently. I’d love to see a wiring diagram or any thoughts of the Pedelecs gurus…

I personally would never recommend connecting two batteries of different age, type and usage. The likelihood of a fire is far too great.
The correct way to do it would be to have a BMS for two batteries, I am told they exist, but I have never seen one myself.
A safer alternative is that you mention, which could be done without relays, provided the switches can handle the current used, which you did not mention...
Possibly using high amp blocking diodes would work, and would prevent the batteries back feeding into the battery with the lowest voltage. Possibly two of these might do the job, one in each battery positive line:-

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But I have never myself tried it out for real.
I have two batteries, and I stop and swap them when the one in use gets too weak. Easy Peasy!
Be careful, this Guy tried it out without proper safety measures and knowledgre and look what happened:-

Remember, the smoke and gases produced are highly toxic.
Basically, you need a pond to sink the bike in to eventually get the fire out, spraying it with water just keeps it from spreading further, it will not fully put the fire outm till all the cells have burnt ot, you can hear each one "popping" in this video!!!
regards
Andy

 

[Nealh]

If one knows what he/she is doing then it not a big deal, it's the dickwhat's using dodgy set ups that are courting trouble. Peronally I think a lot of the ebike scare stories are set ups to look good when they go viral.

I have used lipo & lion in parallel with no issue, I do use alarms on lipo and stop at 3.6v. One puts in the safety measures and doesn't abuse them.

 

[Benjahmin]

I have been running two batteries for about 3 years now. I simply use a two into one lead on the output using anderson plugs.
Discipline is the key.
The batteries MUST be charged disconnected from each other
After charging, measure each battery voltage BEFORE connecting them together.
As Nealh says, they self balance as they discharge. Laws of physics dictate this.
In practice my two chargers go to slightly different levels (0.1-0.15v difference). So, when finished, I use the slightly higher charger to top off the lower battery, they are then equal.
This halves the current drawn from each battery, so stresses them less, important in hilly Wales. It gives me a notional 28Ah capacity.
So long as you are aware of what you're doing and the power you are dealing with and act accordingly, then all will be fine. This is not something that should be done in ignorance.