Charging batteries in parallel

[mrbill]

I often find myself running two or more batteries on my ebike that share the same chemistry but have different pack capacity. When I wire these to my controller I run each of them through a Schottky diode, anode side toward battery positive and cathode toward controller positive, so that one battery doesn't "charge" the other and so that each battery is discharged according to its ability to source current.

Employing a similar concept I'm wondering if I can charge two different capacity batteries that share the same chemistry and charging profile in parallel when those batteries are isolated by diodes, anode side toward charger positive and cathode toward battery positive?

It seems to me this should work without damaging either battery or charger, but since I haven't found any discussion on this forum I thought I'd throw the out the idea in case I've missed something. I do realize that I'd need to accept less than a full-charge into each battery or I'd need to increase the final voltage of my charger to compensate for the diode drop.

My question is motivated by my desire to use one charger to charge two or more batteries simultaneously as I might wish to do overnight when on a multi-day bike tour. I don't want to be awakened during the middle of the night to swap the battery on the charger.

Thanks.

 

[Ypedal]

I would just ditch the diodes all together and just parallel the packs ( assuming we are talking about lithium here and NOT nimh... )

being in parallel, the cell voltages will equalize, if you had a 10ah cell and a 5ah cell in P, it's basically a 15ah cell as far as the charge profile is concerned.

 

[mrbill]

Each battery is a separate sealed unit with its own BMS. I have no access to high-current leads on the individual cells or cell banks inside the batteries. I might also wish to charge different batteries at different states of discharge using this method. In short I'm looking for a clean, fool-proof method that won't result in the release of magic smoke or damage to any component either when I connect the batteries to the charger or during the night while I sleep. I'm ok with getting slightly less than a full charge.

 

[dnmun]

if they have a BMS then if you charge them both in parallel through the C- spot then you have to isolate the two at the P- terminal while charging. you can put the charger on the combined B+ and C- but it has to be isolated at P-. you do not need diodes if they are equivalent packs.

 

[mrbill]

Hi dnmun:

I'm having trouble following your nomenclature.

My batteries are charged and discharged through a common circuit, a single (+) and (-) pair.

Here's a drawing of what I propose to do when batteries are charging when disconnected from the motor controller:

View attachment parallelCharging.pdf

This circuit looks similar to the one I have been using for years when I discharge a similar collection of batteries into a single controller. To visualize that diagram substitute "controller" for "charger" and reverse the direction of the diodes.

Before I hook things up like this I want to make sure I haven't overlooked something.

Thanks.

 

[dnmun]

you do not have a separate C- charging port. you do not need diodes in any form, anywhere.

 

[mrbill]

If I don't use the diodes, then what prevents an uncontrolled current rush (and possible problems such as tripped BMS circuits or worse) when I connect in parallel two batteries at different resting voltages due to different states of charge?

I need a fool-proof protocol that can be relied upon in all situations, because sooner or later I'll do something dumb when I'm not paying attention.

 

[dnmun]

if the battery is inserted in parallel with a resistor on one of the leads then it will limit the current flow between the two packs. then remove the resistor when the voltages are identical and the current stops. the diodes will just consume power and do nothing useful when you connect them in the controller lead too.

 

[mrbill]

OK, so we need a resistor in series on each paralleled battery (minus one) until the voltages equalize.

Do boundary conditions or exceptions cause problems when not using diodes? E.g. BMS trips on one battery but not the other, either during charge or discharge.

How long does it take the batteries to equalize through a resistor chosen to limit current to no more than what the BMS circuit can handle, assuming one battery is full and the other nearly empty? (The goal is to make this fool-proof and simple, so no meters, relays, or switches.)

Let's assume I'm using 7s 3.6v lithium cells, and the BMS for each battery can handle 35A continuous in charge or discharge. Two batteries are placed in parallel, one fully-charge, the other nearly depleted. 29.4v fully-charged, 17.5 volts fully-discharged -> 12 volt difference / 35A = 0.34 Ohms, the smallest resistance that could be used.

How quickly would the voltages equalize? If not within a few seconds I'd need a bulky resistor with a high power rating not to overheat. Can this be done so that the resistor is in-circuit for no longer than a count of "5", preferably less? (Expecting the user to remember to do this and to give the proper count is already going beyond the capability of a fool, but I'm still open to the idea.)

I have a similar arrangement when I connect my battery to my controller to prevent arcing the connectors. But, foolish I occasionally forget to connect through the pre-charge lead, and half the time the BMS in the battery trips from the in-rush current. One time that happened when I was on the road and didn't have my charger with me. The battery would only reset when presented with a charge, so I had to roll the bike backward at about 12mph to generate enough voltage at the battery to reset the BMS. That was harder to do than it sounds.

My diodes lose about 1% but are fool-proof and easy to use. Can you confirm that they are fool-proof when used in the charge direction (even if you don't approve of their use)?

 

[dnmun]

it is not that i don't approve. it is just not needed. if you are connecting them in parallel.

i guess i never expected you would take a discharged battery and tie it in parallel to a charged battery. i figured they would be close in charge so you could stick a resistor in the contacts of the connector.

so you would connect the batteries together at the controller and if one is charged and one is discharged then the diode is there to prevent one from discharging into the other?

so this is just a design meant to use diodes as connectors for batteries "in parallel" that are all either charged or discharged and meant to remain so? essentially all disconnected from each other?

 

[dogman dan]

If you have the diodes, and two chargers, they will charge twice as fast using both chargers. It won't matter even if the chargers work at different rates.

Without diodes, you'd have to unplug them to charge with both chargers.

 

[Ypedal]

do you use only one battery at a time on the bike ? or do you carry both ?

idealy you have both packs on the bike, keep them in parallel full time at the BMS level ( not cell level ) so they are always equal voltage...

if you use one , or the other, at any given time i don't see how this is going to work without having a 2nd charger, one for each pack.

 

[mrbill]

i guess i never expected you would take a discharged battery and tie it in parallel to a charged battery. i figured they would be close in charge so you could stick a resistor in the contacts of the connector.

so you would connect the batteries together at the controller and if one is charged and one is discharged then the diode is there to prevent one from discharging into the other?

so this is just a design meant to use diodes as connectors for batteries "in parallel" that are all either charged or discharged and meant to remain so? essentially all disconnected from each other?

I would not knowingly connect in parallel a fully-charged and fully-discharged battery. But, mistakes happen, and I want fool-proof protocol in case I or someone else does that. If I can't get fool-proof, then I want fault-tolerance such that equipment is not damaged or destroyed if someone (i.e. I) make some dumb-sh** mistake.

I have been discharging two or more dissimilar batteries that share the same chemistry and number of cells through a diode on each battery's (+) lead into a single controller for years. I know that works, is mrbill-proof--I haven't released any magic smoke yet--, and the loss is acceptable and minimal (about 1% in practice).

I'm planning to carry up to four separate batteries and to discharge them in parallel through the diodes as above. If I'm on a tour I only want to carry one charger with me on the bike not four chargers, but I want to be able to charge up to four batteries without having to get up three times in the middle of the night to swap batteries on the charger.

It's a convenience issue. I'm hoping that by charging through diodes, knowing full-well that I won't get a full charge in any battery due to the diode's voltage drop (unless I can increase the charger's termination voltage to compensate), I can get all connected batteries charged to nearly full during the night while I get my beauty sleep. If I still need a full charge, I can manually top off each battery individually in the morning, and the topping-off time for each battery will be short.

I suspect that a side benefit of using diodes is that I may charge in parallel batteries at different states of discharge without causing damage. Each of the batteries will be topped off by the time the charger reaches its termination voltage.

 

[dnmun]

the forward bias on the diode will be insignificant then, but what i was observing was just the loss of power during discharge for no reason. you decided you had to do it this way so you could insert a battery which was not at the same potential so you have created the need for them to solve this problem. of course i know you would never do this since you are so systematic and thorough in action so it did not make sense initially.

you could also use a small 12V light bulb to discharge the difference in potential. then you don't have to carry the big cement power resistor, and the light vanishing would tell you when they were close enuff to tie together too.

about the power loss in the diodes, just picking a Diodes Inc SBR30A60CT from Mouser catalog, 30A, 60V, 700mV@15A $1.98 from this 5 year old catalog.

so if you are pulling around 20A from three packs with a diode on each one, then i would expect on three diodes the forward bias to be around 600mV and power lost would be 12W.

 

[mrbill]

the forward bias on the diode will be insignificant then, but what i was observing was just the loss of power during discharge for no reason. you decided you had to do it this way so you could insert a battery which was not at the same potential so you have created the need for them to solve this problem. of course i know you would never do this since you are so systematic and thorough in action so it did not make sense initially.

you could also use a small 12V light bulb to discharge the difference in potential. then you don't have to carry the big cement power resistor, and the light vanishing would tell you when they were close enuff to tie together too.

about the power loss in the diodes, just picking a Diodes Inc SBR30A60CT from Mouser catalog, 30A, 60V, 700mV@15A $1.98 from this 5 year old catalog.

so if you are pulling around 20A from three packs with a diode on each one, then i would expect on three diodes the forward bias to be around 600mV and power lost would be 12W.

The light bulb concept is reasonable if the packs have not too different SOC and the user doesn't have to wait too long for the batteries to equalize. The problem is that the protocol inserts complexity into the battery->controller or charger->battery connection procedure, and complexity raises the likelihood of user faults. I try to reduce the chance of user fault as much as I can, which is one reason I won't touch the the wiring mess I've seen with the use of small HobbyKing packs in series/parallel.

I could alternately discharge each pack individually, then charge in parallel with diodes using one charger. The tradeoff is more current draw on the single battery resulting in less energy overall from the pack, deeper discharge on the single battery, and the inconvenience of having to swap batteries mid-ride which could necessitate stopping at an inconvenient moment. Running through diodes results in some energy loss, but maybe not as much as placing the load on one pack at a time during discharge. I think it's safe to say that reducing load and depth of discharge will yield a longer battery life, so the my decision tilts toward running in parallel with diodes.

Since all of my battery pack voltages are less than 30V, I'm using the STPS60L30CW from ST Micro, sometimes two diodes in parallel if I'm running only 2 batteries in parallel.

http://mrbill.homeip.net/hybridBike.php#schottky_diode_battery_combiner
http://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/CD00001857.pdf

 

[dnmun]

with a 30V breakdown voltage and 60A doping i bet the forward bias is less than 500mV so the power loss would be about 10W@20A.

 

[mrbill]

with a 30V breakdown voltage and 60A doping i bet the forward bias is less than 500mV so the power loss would be about 10W@20A.

I see a reduction of 0.3V on the CA (which I admit is not terribly accurate reading differences of a few tenths of a volt) after inserting the diode. Vfmax is rated at 0.38V, 0.33V typical for that device. My controller current limit is 45A, so ~22.5A per diode (if using two batteries) or per device if running two diodes in parallel. I know that the small box that houses the diodes never gets more than slightly warm to the touch. In the overall scheme of things the loss is not significant.