Newbie help - twin battery wiring and connectors

[pn_day]

Hi all
I've been lurking for a while, and now have the important approval from the financial controller (aka she who snores in my bed) for buying a kit.

I'm more or less settled on the BMSbattery 500WCST kit, built into a 700C wheel for my Surly Cross Check. Thanks to D8veh for the helpful advice.

However, I'm realising there are lots of things I don't know!

I'm hoping to use 2 batteries (probably 36V 12 or 15 Ah) to balance weight, either in 2 panniers on low riders by the front wheel (as I usually have at least 1 pannier on the back anyway), or in rack top bags at front and rear. Which option would you recommend?

Secondly, when wiring 2 batteries in parallel, do I need to do anything clever on the wiring? I vaguely remember seeing mention of diodes to stop one battery trying to charge the other one if one flattens faster than the other but now can't find mention of it!

Also when charging twin batteries can you charge both in parallel off a single charger (so I can leave one at work and other at home)?

I'm sure I'll think of more questions later...

Thanks
Phil

 

[Deleted member 4366]

Saneagle ran his two batteries in parallel with a single charger. In effect the second battery gets charged through its discharge cables, which has some potential dangers, so probably best to disconnect and charge separately, or get two the same, remove one BMS and parallel the sense wires and the pack power wires to the BMS.

Personally, I think that you'd be better off getting the 20aH shrink-tube battery like mine. It'll give arrange of well over 50 miles (unless you don't pedal) and it's light enough to go behind the seat-post on a rack. As long as you get it close to the seat-post, you won't even notice it's there. Or, there's plenty of room to hang it in your triangle in a bag - DIY or off the shelf.

 

[pn_day]

Thanks for that - I thought 20 Ah total would be a bit tight for range - one way I have just under 20 miles including some big hills. Is that 50 mile range based on flat terrain?
If it is enough this would be much easier to wire in and less obtrusive.
Cheers

 

[Deleted member 4366]

You''ll get about 3 miles per amp-hour over reasonably hilly terrain with medium pedalling at an average speed of about 15mph, which means cruising at 19mph. If you go for maximum speed (24 mph on the road), you could cut that down to 1.5 miles per amp-hour - less if you don't pedal.

 

[Cyclezee]

Hi all
I've been lurking for a while, and now have the important approval from the financial controller (aka she who snores in my bed) for buying a kit.

I'm more or less settled on the BMSbattery 500WCST kit, built into a 700C wheel for my Surly Cross Check. Thanks to D8veh for the helpful advice.

However, I'm realising there are lots of things I don't know!

I'm hoping to use 2 batteries (probably 36V 12 or 15 Ah) to balance weight, either in 2 panniers on low riders by the front wheel (as I usually have at least 1 pannier on the back anyway), or in rack top bags at front and rear. Which option would you recommend?

Secondly, when wiring 2 batteries in parallel, do I need to do anything clever on the wiring? I vaguely remember seeing mention of diodes to stop one battery trying to charge the other one if one flattens faster than the other but now can't find mention of it!

Also when charging twin batteries can you charge both in parallel off a single charger (so I can leave one at work and other at home)?

I'm sure I'll think of more questions later...

Thanks
Phil

Hi Phil,

This probably not useful for you but we have dual battery leads with Anderson connectors for eZee FP batteries which can run in parallel and don't require additional diodes.

 

[trex]

I don't think you need to do anything clever. Anderson lead seems like a good idea.
The batteries cannot be charged through their output port, the stronger battery will get more power drawn until the voltage sag brings both to the same level.
The main danger is fire risk. If one of the two FET switches controlling the output goes, you can't predict the outcome.
I'd would not bother with paralleling two nbatteries. Use one battery until it's pretty flat then stop and switch battrery.

 

[Deleted member 4366]

[quote="trex, post: 212546, member: 5652"
The batteries cannot be charged through their output port, the stronger battery will get more power drawn until the voltage sag brings both to the same level.
[/quote]

I'm not sure if you mean cannot or should not, but to clarify:

The output FETs will remain open as long as the cells are above LVC. It's the charge FET that switches off at HVC, so if you connect the two pairs of output leads in parallel and then charge one battery through its charge port, the voltage of each battery will rise equally to the same voltage until the first one's charge FET switches off when one of it's cells has reached HVC. If the second battery is out of balance, there will be effectively no HVC on the high cells. It's charge FET will switch off the charge port when the first cell reaches HVC, but there's nothing coming in through the charge port. All the charge is pouring in through the output FETS which remain unblocked. The high cells would get over-charged, which could be dangerous, and would definitely be bad for them.

 

[trex]

yes, you are correct with regard to FET switches.
But I remember doing an experiment, I tried to trickle charge a battery through the output port, the BMS switched the output off. I had to open the case and trickle charge through the battery meter.
I can't remember if the same happened when the battery voltage was above LVC but I thought I did repeat the same experiment after tricke charging up to LVC. I don't have an old battery to repeat the experiment now, can't do it with my battery, as there is always the risk of cooking the FET accidentally.

 

[Deleted member 4366]

yes, you are correct with regard to FET switches.
But I remember doing an experiment, I tried to trickle charge a battery through the output port, the BMS switched the output off. I had to open the case and trickle charge through the battery meter.

Was there some other fault that switched off the output FETs?, like the battery or a cell was too low?

 

[trex]

All the cells were down to about 2.4V. I did revive my old rack battery after the cold winter, the BMS worked OK after that. I gave my old bike away when I got my current bike. I don't know if it's still working.

 

[Deleted member 4366]

With cells at 2.4v, both the charge and discharge FETs would be switched off, so you have to go direct to the cells to charge.

 

[trex]

which I did do and succeeded in reviving the pack. Perhaps you can check this out by connecting the output of a charger directly to one of your test batteries, if the LED goes red, we know the FET is on, if it goes green, we know the BMS goes into self defence.
Whichever outcome, it's bad for both batteries if the FETs are both on. Only need one dead cell in either pack to kill both BMSes.

 

[pn_day]

Thanks all - food for thought in all this. If I can get away with just using 1 battery as d8veh has suggested I think this sounds more straightforward for me as one who is new to this whole thing.
Thanks again.

Now the next challenge - moving the pannier rack forward and low enough without fouling the cantilever brake cable. Hmm.