Chargers in series but different current capacity?

[John in CR]

I've used chargers in series for over a year without problem, but they were always identical chargers. Now I need to charge a 24s Konion pack, but the voltage is too low for 3 Bosch chargers to kick on, so I'll have to put unmatched chargers in series. The Bosch chargers put out a little under 4 amps, and the best I can do is add a 24V charger in series that only does 2 amps. Will this cause a problem, or will the string of chargers just revert to the current allowed by the least powerful one?

John

 

[SpeedEBikes]

What is the starting and desired ending voltage for the pack(s) and what are the voltages of the chargers? As long as some voltage from the 2 amp charger is needed then you will be ok - although you won't drive more than 2 amps because the 2 amp charger should reduce it's voltage to limit to 2 amps.

One concern when hooking multiple chargers in series is that their outputs are isolated, ie they are floating with respect to ground (and the other inputs too). Hooking up multiple devices in series that aren't isolated will create an undesired short.

 

[amberwolf]

That latter part would be my first concern--many, if not most, of the cheap chargers I have ever worked with are not isolated. Hooking two of them in series essentially shorts across one of them completely (potentially destroying it and any batteries it's hooked up to, depending on the protection it has against such things).

I've little experience with chargers designed specifically for ebikes, and none with the Bosch tool chargers or most of the others, but regular chargers for many handheld devices, various switching power supplies, etc, are not fully isolated.

 

[John in CR]

Guys,

"Isolated", "floating with respect to ground", etc are terms I don't understand. I've done this with 2 separate but identical pairs of chargers and it worked fine. None of them has a ground plug for the 110v to the wall. I do plug both into the same extension cord. All I do is connect one charger's positive to pack positive, and the other charger's negative to pack negative. Then I just connect the two left over charger wires to each other. I've used this method for 18 months with 2 different identical pairs.

It sounds like I should definitely not try to put my 12V charger for car batteries in series with 2 Bosch chargers, since it seems different in that it had a 3rd ground wire and will come on when seeing 0 volts and not turn itself off, so it works differently.

I do have 5 of the Bosch 36V chargers, so I could try putting 3 in series and use diodes to bring the cutoff voltage down. My concern with that route is that chargers need to see a certain minimum voltage or they won't come on, and I worry that my pack voltage when in need of charging (72v-85v range) may not be high enough to get the chargers to light. I'm unclear if or how the charge cutoff tuning diodes will affect the chargers starting. especially when I need to trim the total down by about 23v.

John

 

[amberwolf]

"Isolated", "floating with respect to ground", etc are terms I don't understand.

Isolated just means that the output is not electrically connected to the input at all, on either positive or negative. If you used a multimeter set to ohms, and measured from either input terminal to either output terminal (with the charger or device not plugged into anything), then there should be no reading at all. If you get continuity in any pair, it's not isolated and shouldn't be tried as series chargers.

Basically, some power supplies and chargers (and DC-DC converters) use their input ground (or sometimes AC neutral) directly or indirectly connected to their output ground (negative output), which means if you connect two of them in series, then the one on the "bottom", with it's positive connected to the negative of the other one, will be shorted across.

If it's already hooked to a battery before it's plugged in (or switched on in the case of switches that disconnect both sides of the input), you'll *also* short out the battery it's hooked up to.

I've done this with 2 separate but identical pairs of chargers and it worked fine. None of them has a ground plug for the 110v to the wall.

That's probably why it works with those, in that they are "floating ground"; they are also isolated inside between input and output.

It sounds like I should definitely not try to put my 12V charger for car batteries in series with 2 Bosch chargers, since it seems different in that it had a 3rd ground wire and will come on when seeing 0 volts and not turn itself off, so it works differently.

It is perfectly possible that it is also isolated, but you'd have to do that ohm test first before hooking it up.

Even if it's safe to hook up, it still won't charge as fast with one lower-current capable supply in it, assuming it has some protection other than a fuse for that overcurrent protection. If all it does is pop the overcurrent protection when too much is passed thru it, then you would have to limit the current from the other two chargers as well, down to whatever the third one can handle. If it's got some physical property of the electronics that prevents more than it's current limit from being drawn thru it (regardless of source) it might work fine without any fiddling around.

I'm unclear if or how the charge cutoff tuning diodes will affect the chargers starting. especially when I need to trim the total down by about 23v.

Maybe put a switch in there that shorts around (some of) the diodes for when you first turn it on to charge, then after it's started the charge process turn the switch off to force it to go thru the diodes.

Better heatsink them well, though, since all the current for the charging will be flowing thru each one.

 

[SpeedEBikes]

So long as you have two chargers with isolated outputs you should be ok hooking up a third that isn't. Don't connect two devices in series that aren't isolated.

I hook up stuff like this all the time and usually get away with it, but it always makes me a little bit nervous, especially if I don't have a thorough understanding of the full range of properties of the devices involved. Are all your devices protected for dead shorts? Can they handle being hooked up with reverse polarity? Over what voltage range? Is the reverse polarity protection going to disconnect the output or short it? How do the devices behave when first plugged in? How do they behave when your AC supply voltage is high or low or has spikes or noise? How do they behave when unplugged while current is flowing? A transformer based unit might have significant inductance which can cause a voltage spike if disconnected abruptly at high current while a switching based unit might not have much tolerance for the spike.

All of which is to say that if you hook the stuff up it may work until it doesn't. When it doesn't work there's a small chance it could fail spectacularly damaging one or more devices or even starting a fire. I wouldn't go to sleep with the stuff hooked up this way unless you torture test it a bit for fault tolerance of all possible operating conditions. If you don't understand the possibilities for failure and the risks you are taking it would be prudent to not do it at all.

 

[317537]

I tried 3x 12v chargers in series it sort of worked, at 48v it didnt, I kept losing one charger. The transformer or an isolation transformer is what isolates the ac voltage. Youre milking AC pulses from a common origin source, transients are going to be present all the time via EMF over the windings and will present as heat. It seemed the more chargers in series I used the more drawing from a common input made a difference. Putting any transformer in series from a common source isnt the best way to get voltage gains.

 

[John in CR]

Thanks guys, input isolated from output makes sense, and really something I should have learned by now.

Speedebikes,
I always put my chargers with a timer protection and always outside, but I hadn't thought about the dirty electricity we have down here, so I'll add an unused voltage regulator into the mix for my little charging station on wheels in the carport. I'll check all my chargers to see which are isolated, and also check the 2 identical looking 36v and 48v chargers I just remembered I have. Maybe I'll get lucky on their cutoff voltage sum, since I've gotten spoiled by the auto cutoff of the Bosches. I went almost a year of needing to be exact with my timer, and I don't want to go back to that, so time to learn more about chargers.

John

 

[monster]

sounds a bit risky to me. i wouldn't risk my battery pack on something like this unless i was absolutly sure it would not damage it.

 

[liveforphysics]

IMO John, it seems like you're asking for the lowest current charger to run up against the possibility of having it's voltage reversed on it, which would shut it down or kill it. As long as the total combined open circuit voltages of the chargers is not above the max safe voltage for the pack, then it shouln't hurt the batteries.

Maybe pick-up a couple of the "meanwell" power supplies off flea-ghey. Some of them have worldwide shipping included in the price, and they are pretty reasonable, and I've found them to be pretty impressive little units.

 

[fechter]

If the chargers are in series, then the current through all of them will be the same. This means the weakest one will be the limiting factor. So when you first start out with a drained pack, the smaller charger will be running at the limit and the larger ones will be sort of loafing. I've used series chargers before with no problems. If you accidentally short the output or lose AC on one charger, one charger could reverse volt the other one and blow it up (don't ask how I know this). As a safety feature, you could install a diode across the output of each charger to prevent reverse voltage. Normally the diode does nothing, but will conduct if the charger output tries to go negative.

 

[John in CR]

I like the diode on each for protection. Since the chargers detect a minimum pack voltage to come on, could the diodes interfere with that? Also, regarding heat sinking the diode string I need to use to get down to the proper cutoff, is that really a concern with the only 3 to 4 amps and only about 1 volt of voltage drop across each?

John

 

[amberwolf]

Also, regarding heat sinking the diode string I need to use to get down to the proper cutoff, is that really a concern with the only 3 to 4 amps and only about 1 volt of voltage drop across each?

Depends on the diodes. Are they designed and spec'd to run at that current and voltage without heatsinking? If you're not sure, heatsink them.

At 4 amps at 1 volt, that's 4 watts. Unless the diodes are the metal-cased kind, or one of the TO- packages with heatsink tabs, they're probably not rated for that kind of power usage. Big block style full-bridge rectifiers out of various switching power supplies (like from computers) probably would be, so you could use those, which would give you essentially two diodes in series for each block. You could probably get away without a heatsink on those.

But the little 1N400x series diodes (and similar body types) might not live long at those current ratings without something to help dissipate the heat. They commonly have very large copper areas on the PCB that they're hooked to when used like that in power supplies, which heatsinks them some. When used at higher currents they usually put them on a vertical heatsink tab with one lead to the PCB thruhole and one lead to the top of the heatsink.

 

[SpeedEBikes]

I like the diode on each for protection. Since the chargers detect a minimum pack voltage to come on, could the diodes interfere with that? Also, regarding heat sinking the diode string I need to use to get down to the proper cutoff, is that really a concern with the only 3 to 4 amps and only about 1 volt of voltage drop across each?

John

One way to do reverse polarity protection with a diode is to wire it reverse biased across the outputs, ie in parallel not serial. This way in normal operation the diode is only bleeding off a tiny leakage current and really not having any effect at all on the output, ie no foward voltage drop. But if you attach something that would reverse the voltage of the outputs you'll now have a forward bias across the diode, essentially a dead short. If the diode is sufficiently large to not vaporize it should fail as a permanent short, ie the diode now just looks like a piece of wire. As long as all your devices have some sort of current limiting or fuses, using diodes this way will protect everything. However, like using fuses for protection diodes used in this way will have to be replaced after each goof up. At least they are quite cheap and easy to test for failure.