Parallel Batteries

[Benjahmin]

My 3-4 year old Ezee battery (I suspect it's pouch cells) is starting to suffer from some serious sag. I have an Insat battery that I want to parallel it with to extend range and stress the both batteries less.
I've made and tested a splitter lead but find I'm nervous of connecting them together for the first time.
Hot off the charger the Ezee is 41.8V, the Insat is 41.9V.
Is this an acceptable voltage difference to allow paralleling them?
Thanks
Ben

 

[Nealh]

Voltage doesn't have to be perfect yours are close enough not to cause any issues, I parallel with in 0.3v total pack voltage.
The only real issue with larger voltages discrepancy is heat transfer through the leads/connectors as big amps will flow from A to B.

 

[Benjahmin]

Thanks Neal, just needed the reassurance.
I've made the splitter with Anderson power poles all round. The common cable leg is 12awg and the two battery legs are 14awg, so don't think there will be a problem with current. I think the Ezee controller is 20A max, though I never use it in max.

 

[Nealh]

The amps flow I mentioned is instant between batteries as they equalise voltage, the flow is rapid and can be mighty. If the unbalance is greater then the larger the amp rush between said batteries. Depending on the cells used it is possible you could 30, 40 or 50a to rapidly flow warming the leads instantly. 12 & 14g for 36/48v is fairly normal and is the same as I use, only once or twice have I had very warm wiring though unequal voltage, once equalisation has taken place they cool down. Never had an issue with melting connectors or wiring though usually limit the discrepancy to 0.3v or less.

 

[anotherkiwi]

I have connected batteries in parallel with 0.1v difference to no ill effect.

15 A x 0.1v = 1.5 W heat generated if I understand current flow from one battery to the other correctly. I use Li-Ion and LiPo in parallel and usually the LiPo is slightly less charged than the Li-Ion and they can handle more current flow than the other way around.

My parallel lead is made by HobbyKing from 16 AWG sillicon wire which is good for 30 A - that means up to 60 A from the two batteries to the controller.

 

[Deleted member 4366]

They don't have to be exact when connecting. Even 0.5v would probably be OK, but you should get them as close as you can.

Whatever you do, make sure that you disconnect them when charging otherwise one will charge the other through the discharge terminals, which can be dangerous, especially when one is worn out.

 

[Benjahmin]

It was because I couldn't work out what the current flow between batteries would be, that I asked the question.

Having a notional 1008Wh to utilise, I couldn't wait to get out there. I've just done a 16mile ride going up one of the longer climbs around.

When I got back combined resting voltage was 38.9. I separated them and, the older Ezee has drifted back up to 39.3, whilst the Insat (Boston cells) has stayed at 38,9. I've separated them because the Insat battery is permanent live to the connectors whilst the Ezee has a key switch. If I left them connected but switched the Ezee off, it could be a bit unfortunate the next time I turn it on.

 

[Danidl]

I have connected batteries in parallel with 0.1v difference to no ill effect.

15 A x 0.1v = 1.5 W heat generated if I understand current flow from one battery to the other correctly. I use Li-Ion and LiPo in parallel and usually the LiPo is slightly less charged than the Li-Ion and they can handle more current flow than the other way around.

My parallel lead is made by HobbyKing from 16 AWG sillicon wire which is good for 30 A - that means up to 60 A from the two batteries to the controller.

Not quite. The current which will flow is voltage difference divided by wire resistance. Say the wire was 0.1ohm and the batteries were different by 0.5v the current which will flow will be 5amps. This will flow until the higher voltage battery has discharged to the level of the lower one.. but the lower one gets some charge. If the wire were 0.01 ohm the current would be 50 amps.

 

[anotherkiwi]

Thanks! Yes I forgot about resistance, been a while since high school physics!

If you are way out of balance between the two batteries then the parallel harness just becomes your fuse...

 

[Benjahmin]

It's exactly the high possible current that bothered me. There's a max. of 8" of 10guage wire between the two batteries, so very low resistance. I'll be aiming for no more than 0.3v difference.
Haven't had chance to try it again yet - weather/work.

 

[Benjahmin]

So I've had a few good rides with both batteries now and it performs well. It's been given a bit of a thrashing on some of the steeper bits hereabouts and the sag is far less. It's nice to know I have the capacity to up the assist level if I get tired on the inward journey. I've got used to the eccentric handling with one battery sitting in a pannier.
The Ezee battery has a switch, the insat battery has none. So, when I plug the paralleled harness into the bikes main harness I get the capacitor charge spark. Not a problem except that i am using Anderson powerpole plugs and I'm a bit concerned that they are getting carbonised. Is there an alternative spark quenching plug or is it not worth bothering with?

 

[Nealh]

XT90's comes in anti spark or a diy route below.
http://scriptasylum.com/rc_speed/nospark.html

 

[Danidl]

XT90's comes in anti spark or a diy route below.
http://scriptasylum.com/rc_speed/nospark.html

I was going to suggest something along those lines.. obviously someone got there before me. The idea of the auxiliary connector is brilliant and could be just as simple as a 3.5mm headphone socket and jack. .. sacrafice a headphone extension lead...a resistor might be problematic, so I might suggest any torch or car bulb.. even a car headlight bulb which has two filiments and needs to be replaced when one fails. , it will briefly flash as the contact is made.

 

[anotherkiwi]

I have been using the same XT60s without spark protection for well over 2000 km. They are a bit black but so far no problems and when there is I'll just solder a new one on.

 

[awol]

Not a problem except that i am using Anderson powerpole plugs and I'm a bit concerned that they are getting carbonised. Is there an alternative spark quenching plug or is it not worth bothering with?

I've been using the same Anderson's with my battery everyday for a few years, they give a little spark everytime so are a bit black but still work ok after all those connect's/disconnects so wouldn't worry about it.

 

[Benjahmin]

Thanks Nealh, does seem a bit of a faff for something that's not going to cause any serious damage. It's just that, as an electrician of some 40 years, I get a bit nervous when I see sparks. It normally means a red face and some vapourised metal from a screwdriver or such.
So I'll stick to the Andersons and just replace if they look too bad, they're cheap enough after all.

 

[Benjahmin]

Lovely weather and got out for a circuit of the hills yesterday. 25miles and got home with 4/5 leds showing. There was no discernable sag on the steepest of climbs (there are a few), so carrying the extra weight seems well worth it. At this rate I can see the original Ezee battery going on for another couple of years.
I've taken the Insat battery out of its wooden case, just using it soft shell in a pannier. Saves about 1kg or so, also allows the battery to sit lower down in the pannier.

 

[Nealh]

If you don't mind or are able to carry an extra battery then paralleling is a good option, it can help an older sagging battery extend it's life and at the same time reduce the load on a new pack. As you have noticed sag is reduced and depending on how far you ride no need to top up straight away as battery may well be at a nice storage voltage.

 

[Benjahmin]

So, last night by mistake, I charged the two batteries whilst the outputs were still connected. Usually the older Ezee charges to 41.3v (two hour 4A charger), and the newer Insat to 41.5 (4 hour 2A charger). However, when I measured this morning the combine voltage was sitting at 41.6v. Just out of curiosity, how has this little piece of magic happened? And what happens to the extra 0.2v when I normally connect together.
I assume it bleeds across in some way, does it help with balancing? Battery circuitry knowledge is a bit sketchy.

 

[wheeliepete]

You will have charged the second battery through the output lead which isn't a good idea, as it bypasses the BMS. You may of slightly over charged one or more cell groups which accounts for the extra voltage. Don't do it again!