Dual Battery Connection Adapter

Hello - I'm thinking of getting a dual battery connection adapter to run dual 36v batteries for extra range. The batteries are both 36v, one newer 25ah & the other older 15ah. Are the cheap adapters to connect them in parallel safe as a way of doing this easily? I plan to disconnect them and charge separately.

Any thoughts or recommended links welcome.

Thanks

You would need to provide details of the specific adapter you mean for anyone to give advice.

Thers are many threads on here on similar topics which the search function might find for you.

The basic principles are to do one of the following:

1. Use two big diodes, one in the positive cable from each battery, joining before the motor connector. This prevents current flow from one battery to the other. This is the safest method.

2. Simply connect the batteries in parallel, but check very carefully that their voltages are within a few tenths of a volt before connecting. Charge the batteries separately and off the bike. This will end badly if you are not careful.

You would need to provide details of the specific adapter you mean for anyone to give advice.

 

Thers are many threads on here on similar topics which the search function might find for you.

 

The basic principles are to do one of the following:

 

1. Use two big diodes, one in the positive cable from each battery, joining before the motor connector. This prevents current flow from one battery to the other. This is the safest method.

 

2. Simply connect the batteries in parallel, but check very carefully that their voltages are within a few tenths of a volt before connecting. Charge the batteries separately and off the bike. This will end badly if you are not careful.

Thanks for the advice.

https://amzn.eu/d/0K7arKj

I was looking at something like this. I run a BBS02 36v with 25A max so I am hoping it would work.

Thanks

Nearly £40 and not clear what is inside. But clearly intended for what you want to do, so should work.

My homemade and rather rough and ready dual diode adapter is an MBR6045WT package containing the two diodes and some connectors soldered on. The MBR6045WT is two 30A diodes, so with batteries at equal state of charge can handle 60A total current, and some guy is selling them at 2 for £2 with free postage on Ebay just now. I just bought two as spares at that price! Never had one fail in over 10,000 miles.

+1 to what [mention=9614]Nealh[/mention] says.

In addition 2x batteries fitted to a bike will be 2x as attractive to an opportunist thief.

the risks of damage to the batteries or worse and the care needed to ensure the batteries are equally charged when connected are all negated at the cost of 30-40 seconds for the exchange (when needed)

Some of us have gone down that dual set up route many years ago, for simplicity one can beat just having them independant and just disconnecting and connecting a fresh battery when needed.

For me the set up was handy to run lion 36v in series with a 3s cell pack for 48v .

In my lipo set up days I would have some 12 or more 6s packs cobbled together in series then paralled together, thankfully Lion cells have improved big time as are the availability of the far better top cells.

I still have a usable 6s and 4s lipo that I wired together as a proper 10s pack and added a 10s BMS with Sw to it so I could charge from a std 2a batteyr charger.

Edited October 20, 20241 yr by Nealh

Two batteries used singularly or as one will have much the same range from my experience .

Pro's and cons, one will have a much larger capacity and voltage will remain higher initially , as one rides deeper in to the discharge then at some stage power output will always slowly decrease.

Seperate batteries will have the same capacity though voltage will reduce quicker, at some stage of the ride one can swap over battery and agian have a fully charged one and benefit from it's extra power for a while.

If one battery is older weaker /aged then there is little benefit from connecting the new one in parallel simply to prop up the weaker one.

Depending on the route use the weaker one for the easier terrain use and save the extra power of the better one for inclines or later in the ride when one might get tired or needs the extra oomph on the way home.

Edited October 20, 20241 yr by Nealh

Thanks for all the advice. Sounds like switching batteries over is the sensible choice. I might try the £2 option suggested by MatthewSlack just to have a play. Looking at the diagram am right in assuming the 2 outside pins are the inputs from the batteries and the middle pin the output to the motor?

Thanks again for the advice.

I might try the £2 option suggested by MatthewSlack just to have a play. Looking at the diagram am right in assuming the 2 outside pins are the inputs from the batteries and the middle pin the output to the motor?

 

Thanks again for the advice.

If you're going to do that, it might be an idea to double them up back to back with a bit of metal plate between them to act as a heatsink, which will reduce any losses and heat by a factor of 4. You get two, so you might as well use them both.

Thanks for all the advice. Sounds like switching batteries over is the sensible choice. I might try the £2 option suggested by MatthewSlack just to have a play. Looking at the diagram am right in assuming the 2 outside pins are the inputs from the batteries and the middle pin the output to the motor?

 

Thanks again for the advice.

Switching batteries is fine, but my bike performs noticeably better when I have two batteries in parallel (with diode separation). The bike can draw more current from the batteries without the voltage slumping.

Switching batteries is fine, but my bike performs noticeably better when I have two batteries in parallel (with diode separation). The bike can draw more current from the batteries without the voltage slumping.

Thanks - that’s one of the benefits I was hoping for. What set up do you use to connect your dual set up?

Peikert's Law says batteries are less efficient at higher currents. According to his law, two batteries in parallel will have greater range than two run separately. I see Peukert's law works when I test capacity of bare cells or completed batteries at different currents,

However, it's not much of an issue when using two 12AH packs since the bike will run efficiently on either. When you use smaller batteries, it shows up and range is increased.

Peumert's Law doesn't address mixing batteries with different discharge characteristics, which can happen when you parallel two different size bike batteries with a dumb y-connector. Results can be surprising. It usually adds up, but I've seen two parallel packs with large difference in capacity give me far less range than their sums would indicate, which argues for using a blender or doing them separate.

Thanks for all the advice. Sounds like switching batteries over is the sensible choice. I might try the £2 option suggested by MatthewSlack just to have a play. Looking at the diagram am right in assuming the 2 outside pins are the inputs from the batteries and the middle pin the output to the motor?

 

Thanks again for the advice.

Exactly that.

All diodes have a threshold voltage before they conduct and this causes a voltage drop across the junction. Germanium type diodes have the least drop at about 0.3v to 0.4v, but silicone ones which are far more common have more - 0.7v is typical. So a battery at 41v connected via a series silicone diode, will present only 40.03v after the diode. Not massive, but a shame to do it unless needed - and of course there is a need if you parallel up the batteries.

I'd be going with the advice up top from nealh when he suggested just connecting them separately. Especially I think as the OP points out the proposed batteries have different age and different capacities.

Some of us never bother with diodes, we simply used th KISS option for day outings..

Charge both batteries to the same voltage within 0.02v and connect up in parallel.

Diodes and the extra wiring etc I can understand if touring esp as Matthew does , as he can't guarantee where or when he may be able to match SOC of his batteries.

Thanks - that’s one of the benefits I was hoping for. What set up do you use to connect your dual set up?

I look after a few bikes for myself and family. Most are Swytch-based, and all feature MBR3045 schottky diode isolation so that we can have an assortment of packs. The earlier post shows you how to wire them - and they are cheap to buy. Make sure you mount them properly on some sort of heatsink with insulation washers, etc.

Some of us never bother with diodes, we simply used th KISS option for day outings..

Charge both batteries to the same voltage within 0.02v and connect up in parallel.

 

Diodes and the extra wiring etc I can understand if touring esp as Matthew does , as he can't guarantee where or when he may be able to match SOC of his batteries.

Mine is only needed for the solar, I have to fool the Shimano into not knowing it is there!

Some of us never bother with diodes, we simply used th KISS option for day outings..

Charge both batteries to the same voltage within 0.02v and connect up in parallel.

 

Diodes and the extra wiring etc I can understand if touring esp as Matthew does , as he can't guarantee where or when he may be able to match SOC of his batteries.

I know that two ALIKE batteries fully charged to the same voltage can be safely connected together and run in parallel, but of course the risk is that someone forgets to make sure the batteries are checked and matched and ends up (careless I know - or ill-informed, but people often are) connecting a half flat battery to a full one and has a very high current flow between them. That is regarded as a high fire risk as you pointed out earlier I think

But what do you think of the idea of connecting a newish 25Ahr and an old 15 Ahr battery in parallel without isolation - or even with it?

Original post at the top here ->

Hello - I'm thinking of getting a dual battery connection adapter to run dual 36v batteries for extra range. The batteries are both 36v, one newer 25ah & the other older 15ah.

I know that two ALIKE batteries fully charged to the same voltage can be safely connected together and run in parallel, but of course the risk is that someone forgets to make sure the batteries are checked and matched and ends up (careless I know - or ill-informed, but people often are) connecting a half flat battery to a full one and has a very high current flow between them. That is regarded as a high fire risk as you pointed out earlier I think

 

But what do you think of the idea of connecting a newish 25Ahr and an old 15 Ahr battery in parallel without isolation - or even with it?

 

Original post at the top here ->

Unless the sag characteristics are identical, every time load is applied and then released there will be a degree of current flow between the batteries.

The common parameter with directly connected parallel batteries is the voltage. Current under load is shared based on pack internal resistance, and if one gives much more than the other there will be a voltage difference when the load is removed, and so a current flow.

You could do some modelling in a spreadsheet to see how much of a problem it is likely to be. In the middle flattish part of the discharge curve probably no big deal, but there might be funny effects at close to empty.

Sorry to be a party pooper, but I must ask...

Why on earth would you want two batteries on your bike???

I use the schottky diodes (since 2018) all the time to parallel up to 3 batteries as I do long runs. I find them more or less foolproof and simple in practice once they are properly installed. The batteries can be left on the bike when charging if you want and the batteries can be charged together with separate chargers or one at a time or in any order. Batteries can be popped in or out if necessary and there is no need to worry about their state of charge or if they balance. There is a little under half a volt loss in the diodes but there is well over a volt gained by the reduction in sag so there is an overall voltage gain in my case.I find it useful with older batteries that have increased their internal resistance and sag badly when used on their own. These older batteries can still give good service when paralleled as the load is shared. So far, so good.....

Diodes are not used with direct drive motors especially with regenerative braking.

Edited October 20, 20241 yr by Sturmey

Unless the sag characteristics are identical, every time load is applied and then released there will be a degree of current flow between the batteries.

 

The common parameter with directly connected parallel batteries is the voltage. Current under load is shared based on pack internal resistance, and if one gives much more than the other there will be a voltage difference when the load is removed, and so a current flow.

 

You could do some modelling in a spreadsheet to see how much of a problem it is likely to be. In the middle flattish part of the discharge curve probably no big deal, but there might be funny effects at close to empty.

That's not right. The current comes out of the batteries. The only way it could go into one is if the other had a higher voltage, but it can't because they're wired together.

It doesn't matter how much any of them sag. Whatever any sag is, it'll be less than the battery on its own because less current will be flowing from each. One cannot sag more than the other when the wires are tied to each other.

That's not right. The current comes out of the batteries. The only way it could go into one is if the other had a higher voltage, but it can't because they're wired together.

 

It doesn't matter how much any of them sag. Whatever any sag is, it'll be less than the battery on its own because less current will be flowing from each. One cannot sag more than the other when the wires are tied to each other.

Yes, the voltage under load stays the same. But a higher proportion of the energy extracted comes from the pack with lower internal resistance, so its position on the discharge curve changes more than the other one.

When the load is removed, the packs therefore want to be at different voltages, and so a current must flow to equalise voltages. Size of current depends on internal resistance of the battery packs.

Yes, the voltage under load stays the same. But a higher proportion of the energy extracted comes from the pack with lower internal resistance, so its position on the discharge curve changes more than the other one.

 

When the load is removed, the packs therefore want to be at different voltages, and so a current must flow to equalise voltages. Size of current depends on internal resistance of the battery packs.

I suppose this must happen inside any battery which has paralleled cells. One of my batteries has a weak group of cells which always loses voltage more than the others. Over time the pack becomes unbalanced even when the bike is out of use. It isn't just that one group is powering the BMS, because even when I get the whole pack properly in balance, it will lose voltage inside ten miles by a couple of tenths of a volt. I reckon that one cell in that group has less capacity, so it is constantly being charged by the others, more or less depending on whether the battery is being loaded or not.