Dual identical batteries in parallel safety/simplicity Questions

[Chuckles1]

I have two UPP 48V 20Ah triangle batteries, both the same model, both with identical 2.5A chargers. I'd like to wire them in parallel for extended range in the simplest, safest manner to power my 48V Bafang rear hub drive bike with the stock controller. Chargers use XTR connectors, if memory serves. Current battery to controller connectors are Anderson powerpoles.

I gather that it may be as simple as buying a Y splitter like ebikes.ca offers for $20. I understand that both batteries should be at the same voltage when the connections are made.

1) Is that all there is to it?

2) How would I best charge the batteries once they are wired in parallel? One charger on either battery, doesn't matter which one? Both chargers at once?

I'd like to avoid any unplugging of the Anderson connectors once they are wired in parallel, as they are very secure but a PITA to plug in when I haven't done it for a while.

 

[amberwolf]

There's a number of good threads about paralleling batteries that discuss the details of why, but basically:

If the BMSs in the packs are "common-port", you can just parallel them with whatever connection method you prefer, as long as they are both at the same voltage at the moment you connect them.

The "safest" manner would be to use one of the "ideal diode" battery combiners, so that if one pack fails for any reason it cannot interfere with the other one by draining it.


If the BMS's are separate charge and discharge ports, you should disconnect the charge ports whenever you are discharging, and disconnect the discharge ports whenever you are charging. If you do not, the BMS cannot protect the cells from overcharge or overdischarge.

 

[tomjasz]

DPDT switch. When one is low flip on-off-on to the other battery. I made a simple display munt out of PVC pipe with a compatible plastic sheet and a $3 volt meter.

 

[Chuckles1]

DPDT switch. When one is low flip on-off-on to the other battery. I made a simple display munt out of PVC pipe with a compatible plastic sheet and a $3 volt meter.

This sounds good; so both batteries are never connected together? That means I could charge both batteries simultaneously with two chargers. Sounds like the only downside is trivial; momentary shut down and restart of display on/off after switching batteries.

I doubt there are any DPDT switches with Anderson powerpole inputs and outputs, so I'll have to fire up the old soldering iron if I go this route. Thanks for the idea!

 

[tomjasz]

DPDT switch. When one is low flip on-off-on to the other battery. I made a simple display munt out of PVC pipe with a compatible plastic sheet and a $3 volt meter.

This sounds good; so both batteries are never connected together? That means I could charge both batteries simultaneously with two chargers. Sounds like the only downside is trivial; momentary shut down and restart of display on/off after switching batteries.

I doubt there are any DPDT switches with Anderson powerpole inputs and outputs, so I'll have to fire up the old soldering iron if I go this route. Thanks for the idea!

See NKK High Capacity Toggles

See series "S" https://www.nkkswitches.com/wp-content/uploads/2022/06/NKK-Switches-Toggles-Catalog.pdf

 

[999zip999]

Soldering Anderson's is ok after a good crimp.

 

[Freddy1]

Maybe a dumb question, but why not simply use one of the 'smart' devices designed to connect two similar voltage batteries of any size together in parallel. The Chinese ones are only about $40 for a 30A model. Come in ranges from 20A to 80A? Physical size gets a little larger the higher the amp rating.

You'll find them on Aliexpress. Have had good experience with everything bought of Aliexpress, as long as you are in no real hurry. Delivery time is slow, about 3 weeks minimum.

 

[tomjasz]

Maybe a dumb question, but why not simply use one of the 'smart' devices designed to connect two similar voltage batteries of any size together in parallel. The Chinese ones are only about $40 for a 30A model. Come in ranges from 20A to 80A? Physical size gets a little larger the higher the amp rating.

You'll find them on Aliexpress. Have had good experience with everything bought of Aliexpress, as long as you are in no real hurry. Delivery time is slow, about 3 weeks minimum.

How smart are they? I have one but haven’t seen or read any testing/tear downs. Until I do it sits in my spares box.

 

[lnanek]

Very little smarts at all. You can Google "ideal diode" if you want a write up. It's just a MOSFET that cuts off current into a battery when the voltages indicate one of the batteries would start charging the other. Not super different from all the MOSFET technology already used in BMSs to limit current.

 

[Freddy1]

Very little smarts at all. You can Google "ideal diode" if you want a write up. It's just a MOSFET that cuts off current into a battery when the voltages indicate one of the batteries would start charging the other. Not super different from all the MOSFET technology already used in BMSs to limit current.

You seem to be suggesting two similar voltage batteries can simply be bolted together in parallel, and the BMS of each battery will manage the situation. That doesn't sound right to me. If these dual battery connecters do more than that, regardless of how technically complicated they are, they would be surely different. If they weren't there would be a market for them.

 

[lnanek]

Maybe English isn't your first language? You suggested using a y adapter with electronics to connect the batteries. E.g. one of these:
https://a.aliexpress.com/_mPhEGiC

Someone else complained they were complex and untrustworthy. I pointed out the circuitry is quite simple and reliable and uses MOSFETs similar to how BMS circuits cut off charging and discharging using MOSFETs:
https://discuss.ardupilot.org/t/ideal-diodes-when-using-multiple-batteries/75487

At no point did I suggest not using one. In fact I have one on my ebike right now. I was pointing out the complaint that they were smart and unreliable and untrustworthy was hypocritical for anyone who already has a BMS on their ebike. Calling it a smart circuit is not correct, however. It's a very simple circuit. There's no chip or programming involved.

 

[tomjasz]

Right, if it's on Aliexpress it's sure to be a good solution.

 

[Freddy1]

Maybe English isn't your first language? You suggested using a y adapter with electronics to connect the batteries. E.g. one of these:
https://a.aliexpress.com/_mPhEGiC

Someone else complained they were complex and untrustworthy. I pointed out the circuitry is quite simple and reliable and uses MOSFETs similar to how BMS circuits cut off charging and discharging using MOSFETs:
https://discuss.ardupilot.org/t/ideal-diodes-when-using-multiple-batteries/75487

At no point did I suggest not using one. In fact I have one on my ebike right now. I was pointing out the complaint that they were smart and unreliable and untrustworthy was hypocritical for anyone who already has a BMS on their ebike. Calling it a smart circuit is not correct, however. It's a very simple circuit. There's no chip or programming involved.

Maybe English isn't your first language, because your first comment was nothing like the second. Yes, in the context of your second response the first takes on a different meaning. However on it's own it was not unreasonable to believe you were supporting the person who had no faith in them.

I give up, this forum seems full of smart arses.

 

[tomjasz]

I give up, this forum seems full of smart arses.

Or maybe just more experience and insights?

 

[E-HP]

Maybe a dumb question, but why not simply use one of the 'smart' devices designed to connect two similar voltage batteries of any size together in parallel. The Chinese ones are only about $40 for a 30A model. Come in ranges from 20A to 80A? Physical size gets a little larger the higher the amp rating.

You'll find them on Aliexpress. Have had good experience with everything bought of Aliexpress, as long as you are in no real hurry. Delivery time is slow, about 3 weeks minimum.

If you can deal with the 0.7V drop across a non-ideal diode, it would cost less than $10 for a couple of diodes to get the same functionality.

 

[Chuckles1]

I have two UPP 48V 20Ah triangle batteries, both the same model, both with identical 2.5A chargers. I'd like to wire them in parallel for extended range in the simplest, safest manner to power my 48V Bafang rear hub drive bike with the stock controller. Chargers use XTR connectors, if memory serves. Current battery to controller connectors are Anderson powerpoles.

I gather that it may be as simple as buying a Y splitter like ebikes.ca offers for $20. I understand that both batteries should be at the same voltage when the connections are made.

1) Is that all there is to it?

2) How would I best charge the batteries once they are wired in parallel? One charger on either battery, doesn't matter which one? Both chargers at once?

I'd like to avoid any unplugging of the Anderson connectors once they are wired in parallel, as they are very secure but a PITA to plug in when I haven't done it for a while.

I appreciate the discussion, but still don't have clear answers to Questions 1 & 2 above. Especially question 2, assuming the answer to question 1 Is "yes."

 

[tomjasz]

1. Yes, not the ideal system, but there will be those that think it's fine.
2. I prefer to charge separately and because I charge most times to 80% that works best for me.

I've been running with the DPDT switch, on-off-on, for a few years. It's simple and safe. I like keeping things simple. I think my way is the safest and most reliable. There are lots of unsubstantiated claims made regularly.

eBikes.ca, Grin, advises...

"One of the easiest ways to increase the current handling capability and range is to put two or more batteries in parallel. In general, with lithium batteries of the same nominal voltage, this is no problem. It is ok to mix old and new lithium batteries in parallel, or even batteries from different manufacturers and with different capacities, so long as they are the same voltage. We stock a parallel battery joining cable to facilitate connecting packs this way.
"
"Where things can get a bit dicey is in charging batteries that are parallel connected. If you leave the batteries in parallel while charging, then the charger current will get shared between the batteries and you can be sure that they are always at the same charge level. However, that does mean one of the batteries will be getting charged through the discharge port. For single port BMS circuits, this is not a concern, but on dual port BMS boards (separate charging and discharging mosfets) there is no overcharge protection on the pack being charged from the discharge leads. This can present a safety risk if there is a cell anomaly in that pack.

Alternately, you can separate the batteries and charge each with its own charger and then connect them in parallel just for discharging. Just make sure that both packs are indeed fully charged, as you don't want to connect them together when one pack is charged and the other is flat.

"

 

[amberwolf]

2) How would I best charge the batteries once they are wired in parallel? One charger on either battery, doesn't matter which one? Both chargers at once?

I'd like to avoid any unplugging of the Anderson connectors once they are wired in parallel, as they are very secure but a PITA to plug in when I haven't done it for a while.

I appreciate the discussion, but still don't have clear answers to Questions 1 & 2 above. Especially question 2, assuming the answer to question 1 Is "yes."

There isn't a simple yes/no.

The answer to question 1 depends on the specific behavior you want, especially in case of various types of failures.

The answer to question 2 depends on what you have/use.

The previous reply here:
https://endless-sphere.com/forums/viewtopic.php?p=1739527#p1739121
answers as clearly as possible #2 without going into excessive detail and without covering every possible derivation/consequence, and addresses the very basic of #1, without going into extreme detail about all the possible failure modes and why they could cause various problems.

If the BMSs in the packs are "common-port", you can just parallel them with whatever connection method you prefer, as long as they are both at the same voltage at the moment you connect them.

The "safest" manner would be to use one of the "ideal diode" battery combiners, so that if one pack fails for any reason it cannot interfere with the other one by draining it.


If the BMS's are separate charge and discharge ports, you should disconnect the charge ports whenever you are discharging, and disconnect the discharge ports whenever you are charging. If you do not, the BMS cannot protect the cells from overcharge or overdischarge.

 

[docw009]

In my opinion, identical batteries can be safely paralleled with a Y-cable as long as you have the discipline to ensure they are at the same voltage. The best way of doing that is when both are at full charged. At 100% charge, a battery is not going to accept much charge current from its mate, even if there is a slight difference in voltage.

For charging, the best practice is to recharge separately. Yes, some batteries have charging circuits that allow them to be charged in parallel. Many do not, and there are situations where it's dangerous.

I think the y-cable with diodes is a good idea, although it's going to waste some heat. It guards against dumb connection mistakes. It also prevents one battery from charging the other one during ebike operation. If the batteries are not identical, and have different series resistances, different AH capacity, this is something that can happen.

 

[john61ct]

Very well stated agree all points there.

Personally, the diodes are only necessary for situations I would not allow.

Parallel charging of separate packs can definitely get dicey, but again NP for those with the knowledge and gear to ensure staying within safe parameters.

For those too ignorant or foolhardy, protective accessories will only protect them so far...

 

[Chuckles1]

Thanks for the latest replies. They really got to the heart of the matter without going over my head. Because electronics has never been my strong suit, I don't know details of my bms, so the possibility of charging safety issues gives me pause. My batteries are UPP brand, so not the highest or lowest quality, from what I gather: dont have a clue what sort of bms they use.

Sounds like the switch is safest way to go, though I'd much prefer a simple "Y" connector if I thought I could charge safely. I wonder if connecting both chargers and switching on both batteries, then activating both chargers simultaneously by using a plug strip with on/off toggle would eliminate the charging safety concerns for a "Y" connector without DPDT switch?

 

[Diggs]

I've seen the parallel adapter from Area13 eBikes (Bolten) used on a few ebikes. Different ah packs are no problem. It works as advertised -

https://www.area13ebikes.com/collections/electrical-accessories/products/dual-battery-parallel-connector

 

[E-HP]

Thanks for the latest replies. They really got to the heart of the matter without going over my head. Because electronics has never been my strong suit, I don't know details of my bms, so the possibility of charging safety issues gives me pause. My batteries are UPP brand, so not the highest or lowest quality, from what I gather: dont have a clue what sort of bms they use.

Sounds like the switch is safest way to go, though I'd much prefer a simple "Y" connector if I thought I could charge safely. I wonder if connecting both chargers and switching on both batteries, then activating both chargers simultaneously by using a plug strip with on/off toggle would eliminate the charging safety concerns for a "Y" connector without DPDT switch?

If you didn't specify the cells for the UPP packs (they have options to specify the cells used, or just get generic Chinese cells), then I'd go with the switch or diode/ideal diode route. If you know what cells are used and that each pack is matched (the two packs could use different cells, but must be matched within the pack) and that they stay in balance through several charge cycles (without balancing), then I'd go for the simple Y connection, to get the advantage of less sag and higher current capability (less stress on the cells). That also allows bulk charging through the discharge port, up to the rated charge current of the cells. (I charge at 8A when in a hurry, and 4A otherwise, for instance).

If the packs can't/won't stay balanced under normal use, then you have to charge separately, and balance the packs regularly, which is a major pain and takes up a lot of time.

 

[tomjasz]

A switch and charging separately with my Satiatorschas been great for 6 years. Initially eith a single Satiator. My newedt batteries are capable of 5A. Great for me. I’m firmly in the camp that believes satiating has greatly increased my battery cycles. Easily making up forbthe investment.

 

[amberwolf]

I don't know details of my bms, so the possibility of charging safety issues gives me pause. My batteries are UPP brand, so not the highest or lowest quality, from what I gather: dont have a clue what sort of bms they use.

Sounds like the switch is safest way to go, though I'd much prefer a simple "Y" connector if I thought I could charge safely. I wonder if connecting both chargers and switching on both batteries, then activating both chargers simultaneously by using a plug strip with on/off toggle would eliminate the charging safety concerns for a "Y" connector without DPDT switch?

It still depends on the BMS design as noted previously--that behavior is what determines what happens in the case of one pack stopping charge for *any* reason while the other continues.

If you don't know which kind of BMS they have, it's possible to check by opening the packs to see if they have separate wiring from the charge and discharge ports to the BMS, so that the BMS has three independent thick wires from it: B- to battery cells, P- to discharge port, C- to charge port.

If P- and C- are the same electrical connection, or there is only a P- with both charge and discharge wires to it, then they are as safe as such things go to directly parallel.

If P- and C- are separate electrical connections, then the problems noted can happen and result in cell damage under certain conditions.