Newbie question: Is this how to wire 72V?

CGameProgrammer

CGameProgrammer

10 kW
Joined
Mar 30, 2007
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645
Location
San Diego, CA
Wiring diagram

Is this a valid way to wire up six 12V batteries in series? I have a 48V charger and a 24V charger. I'm hoping this diagram is good because I won't have to modify the existing 48V wiring at all, besides redirecting the negative output to a positive terminal of the 24V bit.
 
The diagram is a bit confusing because the batteries aren't in the order the voltage is, but at a glance the wiring looks correct. Make sure at least one of the two chargers doesn't have a third ground prong, else the chargers will short, check with a voltmeter as you go, and wear gloves or at least be very, very careful as 72V DC from SLA's pushes enough current to kill, especially if it goes across the chest -- working with one hand can lessen the chance of a fatal zap.
 
Each of the two charger connectors is a three-pronged XLR connector, but the wires that go to it are just one ground and one positive, if I remember right. Is that a problem?
 
No, I have XLRs too, only two of the three prongs are used. Take a look at your charger's plugs that go into the wall outlet.
Two prongs or three?
If, in any chain of chargers like this, more than one is a three-prong, then that can cause a short within those chargers. If that's the case, to make it work the ground prong can be broken off.

You'll notice on the second pic my chain of 20 chargers in series are all two prong chargers plugged into a three-prong extension strip. Since the chargers have two pronged wall-socket plugs, there's no problem.

img_2056_125.jpg
 
I know the 48V charger has a three-pronged plug, and I assume the 24V one does, but they are Soneil chargers and I think ER mentioned that they are 'isolated' and can be used together without a problem.

If they were not isolated, would that simply mean the wiring is fine but that I'd just have to charge one group at a time instead of plugging both chargers in?
 
CGameProgrammer said:
I know the 48V charger has a three-pronged plug, and I assume the 24V one does, but they are Soneil chargers and I think ER mentioned that they are 'isolated' and can be used together without a problem.
Click to expand...

Yes, I've read the Soneil's are an exception in that they're isolated from one another internally, and so even though three-pronged, can be wired to charge in series as you're looking to do.

CGameProgrammer said:
If they were not isolated, would that simply mean the wiring is fine but that I'd just have to charge one group at a time instead of plugging both chargers in?
Click to expand...

If they were not already isolated, then you could either charge one-at-a-time, or break off the ground prong on one or both chargers, thus isolating them.
 
Rather than start a new thread, I'll post another question here: Why isn't the solder sticking to the battery terminals? I tried following ER's soldering instructions by loading the wire end with solder, then pressing it to the terminal and pressing the iron to the wire, letting the solder on the wire melt and pool over the battery terminal. But even though the solder melts onto the terminal, it literally does not stick to it at all! It's frustrating. What could the problem be?
 
What kind of batteries?

The battery terminals take quite a bit more heat to get to soldering temperature than a piece of wire. Try sanding or wire brushing the metal first, then try to tin the battery with a thin layer of solder. Let it cool off, then try soldering the wire on.

I've seen some Ni-Cd batteries that were stainless steel. These are hard to solder to.
 
These are B&B lead-acid batteries. I thought you weren't supposed to heat the terminals for fear of damaging the batteries?

EDIT: Out of hastiness, I went ahead and did the tinning/pre-soldering idea. To minimize risk, I held the spool of solder in the air above the terminal and sort of chopped it with the iron until some drops of solder fell down onto the terminal. Then I let it cool, then tried my previous method for soldering the wires and it worked. Hopefully the batteries are fine. Probably if anything was hurt at all, it's just a slight lifetime reduction... I don't even need to use them for long; they're mainly a high-voltage test before I make the inevitable switch to a chemistry less infatuated with that Peukert guy.
 
72V woohoo!

It works! And each of the two chargers appears to work, though I haven't tried charging both at the same time (only one outlet available). Fully charged voltage is 79.6V. My poor overloaded nylon battery bag now has to deal with 55 lbs of batteries (19 more than before) and I have to be careful carrying it around. My stainless-steel basket that I just dump it into is probably also suffering, but should hold up.

Anyway, acceleration is much better, but since it's nighttime I didn't bother trying for a speed run. I got up to 42 mph but I know the actual top speed will be higher than that -- it was 37.5 mph on the flat with the previous 48V system.

More data tomorrow!
 
Well despite the 50% increase in power, top speed does seem to be 42 mph on flat ground; a change of 13% (from 37 mph). But acceleration is significantly better and range has increased. Fully-charged voltage is a bit over 81V; after an 11.5 mile ride of mostly hard riding today, the voltage was 70V. By that point, my top speed was about 37.

The underside of the controller gets toasty, not quite so hot that I can't keep my hand pressed to it, but definitely very warm. I expected that and don't think anything will get hurt by mere warmth.
 
In light of this thread, along with common sense, I think it's very likely the reason I get such a small speed increase, despite the big acceleration increase, is due to my current limit of 35 amps. I need to increase it.

In fechter's controller modification thread, he said something about the 150V FETs needing to be run at the stock amp limit. Why? They're rated for 79 amps. I'd like to just solder a bridge between two of the parallel shunt wires like fechter once suggested, since that seems really simple.

Of course I'll have to be careful... even at current, er, current, I strongly suspect I hit the overheating sensor, since the controller ramped down current near the end of a 10.5-mile ride last night, and the no-load voltage at that point was 71V. Since the low-voltage cutout hadn't been modified, it certainly wouldn't have triggered that.
 
How does your current draw look at various speeds? My guess is that your power is dropping off due to back emf, and if you're happy with the accelleration, adding more voltage would be the ticket to a higher top speed. Try wiring another 12v battery in series and see what happens.

I've tried 36, 48, 60, 72, 84 and 91v (all nominal) on my motor and even the jump from 84 to 91v is very noticeable.
 
The 150v FETs have about the same on resistance as the stock ones. These are going to get pretty hot even at the stock current limit. Increasing the current will increase the heating.

It might be necessary to increase the airflow or add cooling fins to keep it cool.
 
Lowell said:
How does your current draw look at various speeds? My guess is that your power is dropping off due to back emf, and if you're happy with the accelleration, adding more voltage would be the ticket to a higher top speed.
Click to expand...
I switched from 48V to 72V and my top speed increased by 5 mph. That's disproportionately low, so I figure current is the culprit.

Regarding cooling, this picture shows my controller placement. Wind blows against the bottom of the controller. To improve cooling, do you think the internal components would be damaged (in terms of wind only) by loosening the screws at both ends so there's a 1/8" gap that air can pass in and out of? If the force of the wind might be too strong, I could do things to obstruct it. For example I could tape up this gap except for one section, do something so there's no straight path into it, etc.
 
I switched from 48V to 72V and my top speed increased by 5 mph. That's disproportionately low, so I figure current is the culprit.
Click to expand...

I agree with you, CGameProgrammer. Can you refresh my memory on the system you're running? If you have a 35+ amp controller, there might be something else going on here.....

Then again, 2000 watts to the wheel is needed to maintain 40mph.

http://www.kreuzotter.de/english/espeed.htm

So you need more power somehow to get much above 40mph. Either voltage or current or both should do it...
 
5303, 72V 35A controller (modified with 150V 79A FETs and insulation), 72V 12Ah lead-acid batteries.

The espeed calculator is a bit weird since it seems to try to account for pedaling somehow, but basically it says 2520W (72V 35A) can give a speed of 45 mph, so it's not surprising my actual speed is 42, pretty close really. It also says at 48V 35A my speed would be 40 mph, just 5 mph less, which is indeed exactly what I see. And going to 96V would again only be another 5 mph increase. So I do indeed need to increase current in order to really benefit from the voltage increase.
 
Your setup looks good from a cooling standpoint. I don't think opening up the box will help much. Adding some fins to the face of the controller would help.
 
Fechter, I was worried I was already hitting the temperature cut-off, but upon re-reading your controller thread, it looks like it shuts power off completely? If so, I didn't reach it, which is good. I guess I must have been reaching the low-voltage cutoff, which surprises me given I'm using six batteries, but maybe the voltage under load really was that low... I don't have anything set up to measure voltage while I'm riding.
 
That's possible you hit 29v. The low voltage cutout is soft, like you're describing. Get one of those $2.99 multimeters that Matt has. If your battery voltage is sagging severely during acceleration, it might explain the apparent lack of improvement.

If that's the case, your batteries are probably very unhappy. Do they get hot?

One other thing to consider; as you increase the battery voltage, the amount of current getting to the motor will increase even though the battery current stays at the same limit. This also increases stress and heating on the FETs.

The controller has to get pretty darn hot to shut down (I think 60C), at which time it's like turning off the power switch.

By going to a very high battery voltage, you can keep the battery current lower and get the same power. If your battery pack doesn't support high discharge rates, you can lower the current limit and boost the voltage.
In some cases this might be easier than parallel batteries at a lower voltage.
 
fechter said:
That's possible you hit 29v. The low voltage cutout is soft, like you're describing. Get one of those $2.99 multimeters that Matt has. If your battery voltage is sagging severely during acceleration, it might explain the apparent lack of improvement.
Click to expand...
Acceleration was improved significantly; it's just top speed that only improved by 5 mph. But again, that's actually in agreement with that espeed calculator so hopefully nothing is wrong per se.

If that's the case, your batteries are probably very unhappy. Do they get hot?
Click to expand...
They get warm, but not nearly as warm as the controller, at least not that I've noticed.

By going to a very high battery voltage, you can keep the battery current lower and get the same power.
Click to expand...
Well six lead-acid batteries are already stressful; I'm not going to run eight 9-lb batteries on the back of a bicycle. :) Plus that would be a 33% increase in power, compared to the 71% increase I might get from bridging the shunt. I understand that increasing current is more risky so I'll be taking very brief rides for a bit and checking temperatures. I'll also solder my multimeter to a stripped section of the power cable so I can measure voltage while riding.
 
Plus that would be a 33% increase in power, compared to the 71% increase I might get from bridging the shunt.
Click to expand...

~60 amps from 12ah SLA's I'd think would create very significant voltage sag. Has anybody seen a good discharge curve for various sizes of small, 12 volt SLAs like there are for other the chemistries?
 
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