• I have two bikes each with a TSDZ2 (36v 250w) and a 10s2p battery (Pack A in the pic) that gives me around 45 miles in ECO mode and which I swap between the two bikes as needed.
  
• I am very pleased with the 10s2p battery (self built using twenty LG M50LT 21700 5Ah cells) and so I'm going for a 2nd build (for the second bike) with another twenty M50LT cells.
  
• For the 2nd build I am considering construction two 10s1p batteries (Packs B and C in the pic) and running them in parallel rather than just another straight 10s2p battery.
  
• The reason for considering this is that when I ocassionally want to ride further than the 45 miles that Pack A gives me, I'd 'pinch' Pack B (or C) from the other bike and run it in parallel with Pack A right from the start of the ride which theoretically should boost my range by around another 20 odd miles.
  
• Here's the question
  - apart from Packs B and C each requiring their own BMS, can anyone see any disadvantages in building two 10s1p rather than a single 10s2p pack?
  

Thanks in anticipation of some useful comments.

 

What wiring do you use when you want to connect two of the batteries in parallel ?

What wiring do you use when you want to connect two of the batteries in parallel ?

Paralleling batteries is an oft and well discussed topic and I'm considering a couple of ideas

1. Subject to bench testing and on the understanding that I'll only parallel up when the packs are at the same/similar voltage levels, I'm looking at just using a 'Y' connector - simple, uncomplicated, small and neat

2. Using ideal diodes is another possibility - I have a couple in the work box to experiment with but this approach is not my first choice because it would require some sort of box/container to house the diodes and so forth (more potential failure points). The diodes I have are only rated at 10 Amps as well and although I generally run at half that in ECO mode, there's bound to be the odd ocassion where the 10Amps could be exceeded.

 

I'd considered building in the ideal diodes into the battery pack for neatness but as I'm considering using single port BMS, that wouldn't work. I'd also have to bench test that arrangment to get a feel for the ideal diode heat dissipation.

 

I intend to build the packs with in-line fuses (tut some say and shout 'unnecessary' as that's what the BMS is for) so subject to what inter-pack currents flow when I do my bench testing, I'm currently (no pun intended), I'm leaning towards the simple 'Y' connection.

 

Any views on this approach?

Paralleling batteries is an oft and well discussed topic and I'm considering a couple of ideas

1. Subject to bench testing and on the understanding that I'll only parallel up when the packs are at the same/similar voltage levels, I'm looking at just using a 'Y' connector - simple, uncomplicated, small and neat

2. Using ideal diodes is another possibility - I have a couple in the work box to experiment with but this approach is not my first choice because it would require some sort of box/container to house the diodes and so forth (more potential failure points). The diodes I have are only rated at 10 Amps as well and although I generally run at half that in ECO mode, there's bound to be the odd ocassion where the 10Amps could be exceeded.

 

I'd considered building in the ideal diodes into the battery pack for neatness but as I'm considering using single port BMS, that wouldn't work. I'd also have to bench test that arrangment to get a feel for the ideal diode heat dissipation.

 

I intend to build the packs with in-line fuses (tut some say and shout 'unnecessary' as that's what the BMS is for) so subject to what inter-pack currents flow when I do my bench testing, I'm currently (no pun intended), I'm leaning towards the simple 'Y' connection.

 

Any views on this approach?

Diodes are not really necessary as long as you're sensible and only connect the batteries in parallel when they're approximately the same voltage (fully-charged). In that case you must separate them again before charging. If there's some dunderhead charging and connecting the batteries, who doesn't have a clue about these things, then you need diodes.

Diodes are not really necessary as long as you're sensible and only connect the batteries in parallel when they're approximately the same voltage (fully-charged). In that case you must separate them again before charging. If there's some dunderhead charging and connecting the batteries, who doesn't have a clue about these things, then you need diodes.

I couldn't agree more - I feel confident that I don't fall into that latter category (and I know you weren't suggesting so either, but as we all know, there are dunderheads aplenty in all walks of life - just take a look at www.darwinawards.com to understand that!).

• should boost my range by around another 20 odd miles
  

I am just speaking personally here but I have found on longer runs that I need/like a higher level of assistance. So in my case, my original 400Wh battery was sufficient for 35- 40Km journeys, I now carry well over 1000Wh for 60-80Km rides. I suppose also my batteries (and indeed my leg muscles) have also faded a little over time.

As regards paralleling of batteries, I have found no problem if they are within 1 volt of each other. However, one thing that has happened to me on more than one occasion is that the battery plug on the supplementary battery gets disconnected. I noticed then at about half way through the journey that my main battery is getting depleted. I opened my pannier bag and saw that the second battery connection was pulled out. I then faced a dilemma in that if I plug back in the fully charged second battery, the voltage difference would be large. (I was able to get home by switching out main battery and running on the auxiliary which was adequate to run motor on its own) But I do think the more cells the better when it comes to batteries.

I now generally use diodes but there is a small loss of performance with them (about 1Km/hr) that I notice particularly on one kit that has a low rpm hub motor.

Edited July 12, 20232 yr by Sturmey

I've just ordered a gubbins to use 2 batteries, ali express £21 inc postage. I'm getting a woosh kit for my brompton, already have a spare bag battery for the electric tandem, so intend to put suitable connectors on the 3 bikes so I have options.

 

Ebike Dual Battery Discharge Converter Connection Adapter Switcher 20V-72V 20A/30/40A /80A/120AModule Balanced Discharge

20A XT60

I use abs project boxes and locking speakon connectors with a 20A controller, although they're not rated for high current so I'd look for well under 0.5V between batteries.

 

If one cell loses capacity early to 80%, with 2P the other cell will 'hold it up' i.e. the battery's OK from full to 10%, but with 2 x 1P you'd have batteries fit for 100% and 80%. It probably doesn't matter unless building with used cells, but if weight isn't an issue 2P's more forgiving.

I've just ordered a gubbins to use 2 batteries, ali express £21 inc postage.

In the blurb for that device it says that both batteries need to be the same voltage - what's not clear to me is by 'same voltage' are they just referring to the nominal battery voltage e.g. it's ok to connect two 36v batteries together but not a 36v and a 48v one, OR do they mean the measured battery terminal voltages (which indicate the state of charge) need to be the same?

 

If the latter, then a simple 'Y' cable would do the same thing surely?

If one cell loses capacity early to 80%, with 2P the other cell will 'hold it up' i.e. the battery's OK from full to 10%, but with 2 x 1P you'd have batteries fit for 100% and 80%. It probably doesn't matter unless building with used cells, but if weight isn't an issue 2P's more forgiving.

That's a good point which I'd not considered, thanks.

Thanks [mention=18796]Sturmey[/mention] for your observations and yes, it's a fair point to make about the longer the ride the more tired you'll get and thus you might seek more electrical assistance as the ride progresses.

 

I'm still reasonably fresh after 40 miles or so (that's often including two coffee/cake stops lol ) but we'll have to see what happens with longer rides.

 

What sort of diodes are you using - just ordinary junction diodes with around 0.6v drop, or ideal diodes with around 0.1v drop?

Get some ideas from this. I can't remember the guy mentioning diodes:

https://www.electricbike.com/dewalt-cordless-battery-cruiser/

Used a simple Y splitter on two batteries for about 5 years now.

I ALWAYS measure the voltage of the two packs before connecting together. I will not connect if the difference is above 0.5v.

Although both packs have their own chargers, I use the same one to finish both packs, so not normally a problem.

If they do become seprerated during a ride (it has happened)- DON'T RISK IT ! Deplete one pack then swap to the other.

With a bit of discipline and understanding, it works well.

In the blurb for that device it says that both batteries need to be the same voltage - what's not clear to me is by 'same voltage' are they just referring to the nominal battery voltage e.g. it's ok to connect two 36v batteries together but not a 36v and a 48v one, OR do they mean the measured battery terminal voltages (which indicate the state of charge) need to be the same?

 

If the latter, then a simple 'Y' cable would do the same thing surely?

 

Nominal voltage needs to be the same, one shouldn't mix nominal voltages .

Nominal voltage needs to be the same, one shouldn't mix nominal voltages .

I'm trusting that that is the case, cheap enough mistake if not. I just saw something very similar on fleabay for £139!

A diode pack certainly isn't essential but it would be cheap, simple and safe, it would address your remaining questions and suit your application. Just mount it in fresh air for an easy life. With a Y cable you can only join up if the measured Vs are close... skip all that fuss

Get some ideas from this. I can't remember the guy mentioning diodes:

https://www.electricbike.com/dewalt-cordless-battery-cruiser/

That's a great piece of fabrication by Blair - whilst I've only glanced through some of the vids, the only comment by the reviewer (Ron / Spinning Magnets of ES fame) relating to battery paralleling mentions ensuring the battery voltages are equal when replacing any one battery as there are 3 pairs of batteries making up the total.

 

So as you say, no mention of diodes - Room 101 for them then

Edited July 13, 20232 yr by Bikes4two

Used a simple Y splitter on two batteries for about 5 years now.

I ALWAYS measure the voltage of the two packs before connecting together. I will not connect if the difference is above 0.5v.

Although both packs have their own chargers, I use the same one to finish both packs, so not normally a problem.

If they do become seprerated during a ride (it has happened)- DON'T RISK IT ! Deplete one pack then swap to the other.

With a bit of discipline and understanding, it works well.

I think I agree 100%, but in case I misunderstood what you meant, it's absolutely critical that the two batteries are not connected together when you charge either one or both. They must be charged separately and disconnected from each other for charging otherwise one will charge the other through the discharge port, which bi-passes the charge safety controls, which is definitely dangerous, not just some theoretical risk.

Agreed, and reading between the lines Bikes4two might have a dual port BMS on his first battery. But IF both batteries were known to be common charge/discharge port, should it then be OK to charge them in parallel, hypothetically? I'm unsure what puts a common port into charge mode.

 

I imagine it would still be unwise because a) BMS's not matching could lead to one never balancing and b) BMS's seem to fail quite often.

Agreed, and reading between the lines Bikes4two might have a dual port BMS on his first battery. But IF both batteries were known to be common charge/discharge port, should it then be OK to charge them in parallel, hypothetically? I'm unsure what puts a common port into charge mode.

 

• I look forward to hearing comments on this as at the moment my primary pack (Pack A) has a 'same port' BMS whilst I have yet to decide on what BMS for the Packs B and C although my gut feeling is that separate charging/chargers are needed.
  
• I'm definitel new to this game (of ebike batteries etc, but not elecrical/electronic engineering in general though)
  
• If my basic understanding of the CC/CV charging method is correct(?), then you'd need to charge each pack separately otherwise I can see that the charger switchover point from going Constant Current to Constant Voltage might be at the correct point for one battery pack but not the other.
  
• And if the BMS were using passive balancing then this in turn would leave the balancing incomplete in one of the battery packs.
  

 

(Happy to be corrected on what I've just said - every day's a school day and I'm still learning for sure).

Edited July 13, 20232 yr by Bikes4two

And these sort of adapters, if robust enough, might well come in handy for straight paralleling.

 

I'm getting one from ebay to check out the build quality but fabricating my own is an option, but at £3.76 inc posting, I'll try a bought one first.

I ALWAYS seperate the batteries for charging because I have one 4A charger and one 2A.

However, even if they were the same amperage and make, there are too many variables.

Terminal voltage setting could be adrift by 0.1-0.2v or more.

As they heat up during charging component drift will differ.

Switch over point may not coincide.

 

Any charging through the discharge port bypasses the BMS, a recipe for potential horror story.

 

On a couple of occaisions over the years I have mistakenly left the Y splitter connected:eek: and had a, correct and needed, right royal bollocking from this forum.

 

So, even with diodes, I would seperate, 'cos things have been known to break down if my past experience with zenor diodes on old British motorbikes is anything to go by. I know it does a different job but point made.

One thing to remember when using a "Y" lead or double connector: When you plug in the first battery, the second connector has live male pins that can short if they touch metal.

BMS

One thing to remember when using a "Y" lead or double connector: When you plug in the first battery, the second connector has live male pins that can short if they touch metal.

True enough which is why I like to use BMS modules that have an on/off switch to mitigate the risk of doing that.

One thing to remember when using a "Y" lead or double connector: When you plug in the first battery, the second connector has live male pins that can short if they touch metal.

Why can't the connectors be fitted such that the live side is female?