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Ran D. St. Clair · 02-02-2010, 07:50 PM

I Was Wrong (Again)

I was doing some testing on my electrical system. It was pretty much my first chance to do so with everything in place. I tried various AC loads up to about 2,300W, which is getting close to the limit of my inverter (2,500W) and everything was working as expected. As a result of the testing I had managed to draw my batteries down a bit and my Battery Charger was pumping its full rated 90A from shore power. I decided to try my Honda Eu2000i generator instead and it fired up happily and was pumping 9.5A at 124VAC into the charger, so all was well. As I continued to try stuff the batteries were coming back up to 90% charge and I happened to apply a smaller AC load, something like 1,500W. My light plugged into the inverter started flickering, and my generator ramped up to full throttle for a couple of seconds before giving up and cutting its AC output.

My first reaction was, Oh crap, followed immediately by, what the hell? My generator is happy with a heavy load, but doesn’t like a light one? The flickering light was my best clue. I remember back about 6 months ago when I did my first testing with the inverter and the Power Supply (Battery Charger) but no batteries. I noticed the flickering then and assumed that it was because I didn’t have any batteries to stabilize the DC side of things. I now have about 400 pounds of batteries so that is no longer an issue. I also remembered that the flickering went away when I unplugged the little module that converts my Power Supply into a 3 stage battery charger. I unplugged the module again, and sure enough everything started working as expected.

It seems my battery charger was hunting, trying to decide whether it should be in Float, Absorption, or Bulk mode (I don’t know exactly which). This was pushing the DC voltage at the batteries up and down slightly which was in turn causing the flicker at the inverter output. It was also flickering the current draw from my generator, which really pissed it off.

When my batteries were more depleted, the battery charger wasn’t confused about what stage it should be in. It was happy to stay in bulk mode, and just push 90A. This made the inverter and generator happy as well. Even as the batteries came up to full charge, a really heavy load was enough to convince the battery charger to remain in Bulk mode, so everyone was happy. It was only when the batteries were more fully charged and the load was reduced that the charger wasn’t sure what to do.

Without the module my power supply is still a battery charger, but it only has one stage, “float”. In this case, float means that it will try to bring the batteries up to 13.56V, and will give them all the current they will take at that voltage, up to a maximum of 90A. If the batteries will take the full 90A then the voltage has to drop to whatever the battery voltage is with 90A input. Depending on the battery state of charge, that will be between 11V and 13.56V. To put all this another way, my battery charger is really a 13.56V power supply with a 90A current limit.

With the module installed my battery charger has 3 stages:

Bulk 14.67V at up to 90A
Absorption 14.16V at up to 90A
Float 13.56V at up to 90A

All three stages work the same way. They put out the specified voltage unless limited by the 90A current limit. The idea is that you can safely charge a heavily discharged battery at a higher current, which means you can safely push that higher current with a higher voltage. As the battery approaches full charge you need to back off the voltage to avoid overcharging the batteries which would shorten their life.

This is all a fine idea, and well proven, but it really doesn’t apply to my situation. I have a large number of high quality AGM (Absorbent Glass Mat) batteries. If I were to run them down to say 50% charge, and then hit them with a bulk charge voltage of 14.67V and no current limit, they would probably happily accept 600A or more. This would indeed be the most time efficient way to charge my batteries, but I simply don’t have that kind of power available. My generator isn’t powerful enough, and my battery charger is intentionally sized so as not to overload my generator.

In my case it doesn’t matter if the battery charger is trying to push 13.56V, or 14.16V, or 14.67V, the current is going to be limited to 90A. As the batteries come up to full charge there will eventually be a point at around 90% of charge, where the 90A limit will no longer apply. At this point my charger should be in Float stage anyway, and limiting the voltage to 13.56V. As it turns out, I don’t need a 3 stage charger. A one stage or a 3 stage charger will both do almost exactly the same thing, and charge my batteries equally quickly.

So, I was wrong about needing a 3 stage battery charger, and I wasted $29 on that module. As mistakes go, this was a cheap one…

It turns out that I was also at least partly wrong about one other thing. I designed my truck so the generator would drive the battery charger and only the battery charger. The idea was that any sudden increase in load would be provided by the batteries so the generator would never see a sharp increase or decrease. This would allow me to run the generator in ECO mode where it throttles down to provide only the power that is needed.

My plan works perfectly so long as the batteries are below about 90% of charge. In that case the battery charger is pumping the full 90A which means the generator is happily pumping about 80% of its rated capacity, and all is well. If, however, the battery is nearly fully charged, then the battery charger isn’t charging much and the load on the generator is very small. In that case, a sudden increase in load, like turning on the air conditioner, is transmitted almost immediately all the way back to the generator. Fortunately, even in ECO mode at full idle, the generator seems to ramp up quickly enough to avoid anything bad happening. The batteries are probably taking the edge off the surge, and both the generator and the battery charger are willing to deal with low voltage for a second or two until everything has a chance to stabilize. It’s not ideal, but so far it all seems to be acceptable.

To be continued….

02-02-2010, 07:50 PM	#150
Ran D. St. Clair Senior Member Join Date: Apr 2009 Posts: 212	I Was Wrong (Again) I was doing some testing on my electrical system. It was pretty much my first chance to do so with everything in place. I tried various AC loads up to about 2,300W, which is getting close to the limit of my inverter (2,500W) and everything was working as expected. As a result of the testing I had managed to draw my batteries down a bit and my Battery Charger was pumping its full rated 90A from shore power. I decided to try my Honda Eu2000i generator instead and it fired up happily and was pumping 9.5A at 124VAC into the charger, so all was well. As I continued to try stuff the batteries were coming back up to 90% charge and I happened to apply a smaller AC load, something like 1,500W. My light plugged into the inverter started flickering, and my generator ramped up to full throttle for a couple of seconds before giving up and cutting its AC output. My first reaction was, Oh crap, followed immediately by, what the hell? My generator is happy with a heavy load, but doesn’t like a light one? The flickering light was my best clue. I remember back about 6 months ago when I did my first testing with the inverter and the Power Supply (Battery Charger) but no batteries. I noticed the flickering then and assumed that it was because I didn’t have any batteries to stabilize the DC side of things. I now have about 400 pounds of batteries so that is no longer an issue. I also remembered that the flickering went away when I unplugged the little module that converts my Power Supply into a 3 stage battery charger. I unplugged the module again, and sure enough everything started working as expected. It seems my battery charger was hunting, trying to decide whether it should be in Float, Absorption, or Bulk mode (I don’t know exactly which). This was pushing the DC voltage at the batteries up and down slightly which was in turn causing the flicker at the inverter output. It was also flickering the current draw from my generator, which really pissed it off. When my batteries were more depleted, the battery charger wasn’t confused about what stage it should be in. It was happy to stay in bulk mode, and just push 90A. This made the inverter and generator happy as well. Even as the batteries came up to full charge, a really heavy load was enough to convince the battery charger to remain in Bulk mode, so everyone was happy. It was only when the batteries were more fully charged and the load was reduced that the charger wasn’t sure what to do. Without the module my power supply is still a battery charger, but it only has one stage, “float”. In this case, float means that it will try to bring the batteries up to 13.56V, and will give them all the current they will take at that voltage, up to a maximum of 90A. If the batteries will take the full 90A then the voltage has to drop to whatever the battery voltage is with 90A input. Depending on the battery state of charge, that will be between 11V and 13.56V. To put all this another way, my battery charger is really a 13.56V power supply with a 90A current limit. With the module installed my battery charger has 3 stages: Bulk 14.67V at up to 90A Absorption 14.16V at up to 90A Float 13.56V at up to 90A All three stages work the same way. They put out the specified voltage unless limited by the 90A current limit. The idea is that you can safely charge a heavily discharged battery at a higher current, which means you can safely push that higher current with a higher voltage. As the battery approaches full charge you need to back off the voltage to avoid overcharging the batteries which would shorten their life. This is all a fine idea, and well proven, but it really doesn’t apply to my situation. I have a large number of high quality AGM (Absorbent Glass Mat) batteries. If I were to run them down to say 50% charge, and then hit them with a bulk charge voltage of 14.67V and no current limit, they would probably happily accept 600A or more. This would indeed be the most time efficient way to charge my batteries, but I simply don’t have that kind of power available. My generator isn’t powerful enough, and my battery charger is intentionally sized so as not to overload my generator. In my case it doesn’t matter if the battery charger is trying to push 13.56V, or 14.16V, or 14.67V, the current is going to be limited to 90A. As the batteries come up to full charge there will eventually be a point at around 90% of charge, where the 90A limit will no longer apply. At this point my charger should be in Float stage anyway, and limiting the voltage to 13.56V. As it turns out, I don’t need a 3 stage charger. A one stage or a 3 stage charger will both do almost exactly the same thing, and charge my batteries equally quickly. So, I was wrong about needing a 3 stage battery charger, and I wasted $29 on that module. As mistakes go, this was a cheap one… It turns out that I was also at least partly wrong about one other thing. I designed my truck so the generator would drive the battery charger and only the battery charger. The idea was that any sudden increase in load would be provided by the batteries so the generator would never see a sharp increase or decrease. This would allow me to run the generator in ECO mode where it throttles down to provide only the power that is needed. My plan works perfectly so long as the batteries are below about 90% of charge. In that case the battery charger is pumping the full 90A which means the generator is happily pumping about 80% of its rated capacity, and all is well. If, however, the battery is nearly fully charged, then the battery charger isn’t charging much and the load on the generator is very small. In that case, a sudden increase in load, like turning on the air conditioner, is transmitted almost immediately all the way back to the generator. Fortunately, even in ECO mode at full idle, the generator seems to ramp up quickly enough to avoid anything bad happening. The batteries are probably taking the edge off the surge, and both the generator and the battery charger are willing to deal with low voltage for a second or two until everything has a chance to stabilize. It’s not ideal, but so far it all seems to be acceptable. To be continued….