MEP-501a Question

[JR2980]

So I recently acquired a Mechron version of the MEP-501a (ASB 2D/DND6) - I really didn't expect to get it for the price I was willing to pay, but hey - I'll take it!

Now I have a few questions that I'm hoping someone here can help me with.

I want to use the genset to recharge a 24v battery bank, but have some concerns about how to connect it.

1 - its rated at 28v, and I think I remember reading that its adjustable down to 26.x volts. Will that be an issue for charging a 24v system? I seem to remember that full charging voltage for a 24v system was just under 26v.

2 - do I need a charge controller? There is a section of the manual that talks about the output and how there is a "sense" wire that senses and regulates the output, but if I hook it directly up to discharged batteries, they could easily overload the genset..... right? Or am I missing something?

3 - assuming that I need a charge controller, what kind? Most of the ones out there that I've seen are for solar, and I'm not sure that they will work correctly for this application. I did contact Xantrex about their C60 charge controller - they said it wouldn't work, but the reason that they gave seemed to me to be BS. He said that the controller was for solar panels only because it could tell the solar panels to throttle down if it didn't need the power, but it couldn't tell the genset to throttle down, so it would burn up. Maybe I'm missing something, but I didn't think there was a way to "throttle down" solar - doesn't a solar panel provide power as long as there is sunlight on it, with the amount of sunlight determining the power output??? I though that the charge controllers (at least some of them) would divert to a dummy load if needed, but didn't know that there was a way to "throttle down" the solar panel. It seems like it would work for my application since the genset will sense the load and adjust accordingly - right?


I'd like to use the full (or as close to it as reasonable) 69 amp output from the genset to charge the batteries so that they will charge as quickly as possible so that I dont have to run the genset too long. I was thinking maybe a 60amp charge controller, and I also have an inverter that I can connect to get AC power off of it if needed.

Anyone done this before, or can tell me if I'm way off base here? Definitely don't want to tear up the genset or my batteries!



Thanks!

 

[Rapracing]

I believe "full charge" on a 24v system is around 25.5 or something like that. You can charge at the 28 volts i do believe without an issue.

 

[DieselGenAC]

1 - its rated at 28v, and I think I remember reading that its adjustable down to 26.x volts. Will that be an issue for charging a 24v system? I seem to remember that full charging voltage for a 24v system was just under 26v.

A typical max charging voltage for a "12V" lead acid or AGM battery is 14.1V so a 28V genset is perfect for two 12V batteries in series.

2 - do I need a charge controller? There is a section of the manual that talks about the output and how there is a "sense" wire that senses and regulates the output, but if I hook it directly up to discharged batteries, they could easily overload the genset..... right? Or am I missing something?

I don't know about any sense wire in the 501A but I do know that you are not going to get more than 2.2 to 2.4kW out of it. If your batteries draw more than 80 or 90 amps, the breaker will trip or the unit will bog. If the voltage drops off too far, bad stuff happens as well because the generator controller will think it needs to reflash the alternator and will fire off field current to the alternator.

3 - assuming that I need a charge controller, what kind? Most of the ones out there that I've seen are for solar, and I'm not sure that they will work correctly for this application. I did contact Xantrex about their C60 charge controller - they said it wouldn't work, but the reason that they gave seemed to me to be BS. He said that the controller was for solar panels only because it could tell the solar panels to throttle down if it didn't need the power, but it couldn't tell the genset to throttle down, so it would burn up. Maybe I'm missing something, but I didn't think there was a way to "throttle down" solar - doesn't a solar panel provide power as long as there is sunlight on it, with the amount of sunlight determining the power output??? I though that the charge controllers (at least some of them) would divert to a dummy load if needed, but didn't know that there was a way to "throttle down" the solar panel. It seems like it would work for my application since the genset will sense the load and adjust accordingly - right?

The 501A voltage regulator is a fixed voltage unit. It will try to hold 28V no matter what, even at the expense of overloading the engine. You might be on to something with the solar charge controller. I think a solar charge controller works by converting DC power from higher voltages off the PV panels to the lower battery voltage, usually with high frequency switching. There are losses and efficiencies involved so most DC-DC converters like this need an input voltage a bit higher than the nominal output voltage so you might have to crank up the generator voltage a few volts to get 28V out of the charge controller. I do not know how high the voltage can be adjusted in the 501A.

I'd like to use the full (or as close to it as reasonable) 69 amp output from the genset to charge the batteries so that they will charge as quickly as possible so that I dont have to run the genset too long. I was thinking maybe a 60amp charge controller, and I also have an inverter that I can connect to get AC power off of it if needed.Anyone done this before, or can tell me if I'm way off base here? Definitely don't want to tear up the genset or my batteries! Thanks!

Sounds like a good project. Keep us updated.

 

[JR2980]

A typical max charging voltage for a "12V" lead acid or AGM battery is 14.1V so a 28V genset is perfect for two 12V batteries in series.I don't know about any sense wire in the 501A but I do know that you are not going to get more than 2.2 to 2.4kW out of it. If your batteries draw more than 80 or 90 amps, the breaker will trip or the unit will bog. If the voltage drops off too far, bad stuff happens as well because the generator controller will think it needs to reflash the alternator and will fire off field current to the alternator.

I worded that wrong - the fully charged (at rest) voltage of a 12v battery is something like 12.7v, which would put the at rest voltage for 2 fully charged 12v batteries in series at 25.4v, which is lower than the lowest voltage I can set the genset to......so hooking them directly to the genset I would run the danger of overcharging and damaging the batteries unless I stand there with a meter and cut the genset off as soon as they're fully charged....which I don't really want to do

The load sense is talked about in the manual here.... at the bottom of page 58 http://www.deweyelectronics.com/ass...-Tactical-Generator-Sets-Technical-Manual.pdf

The 501A voltage regulator is a fixed voltage unit. It will try to hold 28V no matter what, even at the expense of overloading the engine. You might be on to something with the solar charge controller. I think a solar charge controller works by converting DC power from higher voltages off the PV panels to the lower battery voltage, usually with high frequency switching. There are losses and efficiencies involved so most DC-DC converters like this need an input voltage a bit higher than the nominal output voltage so you might have to crank up the generator voltage a few volts to get 28V out of the charge controller. I do not know how high the voltage can be adjusted in the 501A.

Voltage range is 26.6v - 32.2v per the manual. (page 59)

I mainly want to use the charge controller with multi stage charging so that it will do the "bulk" charge at high amp, then step down to "top off" stage, then to "float" or maintenance stage so as not to damage the batteries.

Sounds like a good project. Keep us updated.

Will do, but I have alot of research to do - batteries aren't cheap and I'm real leery of damaging my batteries, and for that matter, a good charge controller isn't cheap either. I may get a cheap charge controller and do some testing if I can find a couple of old batteries and see how the system works together.



Thanks!

 

[DieselGenAC]

I worded that wrong - the fully charged (at rest) voltage of a 12v battery is something like 12.7v, which would put the at rest voltage for 2 fully charged 12v batteries in series at 25.4v, which is lower than the lowest voltage I can set the genset to......so hooking them directly to the genset I would run the danger of overcharging and damaging the batteries unless I stand there with a meter and cut the genset off as soon as they're fully charged....which I don't really want to do

Yes, that is the resting voltage. You need to go a little higher than that to push the chemical reaction inside the battery to charge. For example, a typical automotive charging system runs at 13.8 to 14.4V to charge it's onboard 12V battery.

I mainly want to use the charge controller with multi stage charging so that it will do the "bulk" charge at high amp, then step down to "top off" stage, then to "float" or maintenance stage so as not to damage the batteries.Will do, but I have alot of research to do - batteries aren't cheap and I'm real leery of damaging my batteries, and for that matter, a good charge controller isn't cheap either. I may get a cheap charge controller and do some testing if I can find a couple of old batteries and see how the system works together.Thanks!

I'm assuming a 60A charge controller automatically limits OUTPUT to 60A (60A X 28V = 1680W out of the charge controller). With losses, that is probably around 2000W generator output to the controller depending on efficiency. If the efficiency is worse that that, you might have to run a 45A charge controller. If the charge controller does not limit current, then all bets are off and you may overload the generator. I doubt any manufacturer will understand how their stuff will work between batteries and a fixed voltage DC generator so you will have to experiment on your own. The good news is that solar charge controllers are built to charge batteries so you won't hurt those expensive boxes of lead and acid.

 

[JR2980]

I'm assuming a 60A charge controller automatically limits OUTPUT to 60A (60A X 28V = 1680W out of the charge controller). With losses, that is probably around 2000W generator output to the controller depending on efficiency. If the efficiency is worse that that, you might have to run a 45A charge controller. If the charge controller does not limit current, then all bets are off and you may overload the generator. I doubt any manufacturer will understand how their stuff will work between batteries and a fixed voltage DC generator so you will have to experiment on your own..

That's pretty much what I was thinking too - I was just hoping that someone could confirm that for me.

I'm thinking of spending $30 on a 30 amp charge controller - it will either limit the output (and if I understand the manual, the genset will sense the load and only deliver what the charge controller needs), or the charge controller will get the full 70+ amps, and go up in smoke long before I overload the genset (at least I hope so). Worst case, I cook the controller and I'm out $30 for a cool lightshow when it goes up.....best case, it works and I have an extra charge controller sitting around to add to all the other junk - I mean useful stuff - that I have sitting around (my wife will LOVE that).


If anyone else has any input or relevant experience here, it would be appreciated!


Thanks!

 

[steelypip]

Charging voltage: The best way isn't to constant voltage charge anyway. It'll do in a pinch, but wet cell lead acid batteries benefit from a certain amount of 'overcharge' or 'equalization charge' at the end of a charging cycle once in a while to stir the electrolyte (not a problem in a vehicle) and even out the deposition on the plates. This can be difficult to get right without the ability to nudge the voltage/cell to 2.5 or so.

For longest battery life, your peak charge rate should be c/10 or less, where c is the amp-hour capacity of the battery. Allowing for inefficiencies in charging, this can mean a 15+ hour charge if the battery bank is completely flat. Continuous charging faster than c/5 requires monitoring battery temperature to prevent excessive electrolyte loss.

The charge controller might be fine or it might burn up. It depends on whether it shunts excess source current to ground (shunt regulator) or increases the resistance between the source and load to reduce current (series regulator). A series regulator will be fine. A shunt regulator will fry itself fighting the generator regulator. Good charge controllers have more complicated charging cycles than simple constant voltage charging, and can be programmed to do a lot of clever things like manage rate of charge by bank temperature and adjust cutoff voltage depending on battery bank health and equalization.

Lastly: why are you using 24 volts for the bank? Higher voltages are always more efficient, and cheaper to wire.

 

[DieselGenAC]

or the charge controller will get the full 70+ amps, and go up in smoke long before I overload the genset (at least I hope so). Worst case, I cook the controller and I'm out $30 for a cool lightshow when it goes up.....

My best guess is that your test will be successful. Roll video anyway to capture any chance of copper turning into glorious flames and smoke.

 

[JR2980]

Thanks for the input - sounds like you know alot more about this than I do!

Charging voltage: The best way isn't to constant voltage charge anyway. It'll do in a pinch, but wet cell lead acid batteries benefit from a certain amount of 'overcharge' or 'equalization charge' at the end of a charging cycle once in a while to stir the electrolyte (not a problem in a vehicle) and even out the deposition on the plates. This can be difficult to get right without the ability to nudge the voltage/cell to 2.5 or so.

That could potentially be a problem since this genset maintains constant voltage - I can vary the voltage between 26.6v and 32.2v, but it wont vary automatically.

I did get another response back from Xantrex about the C60 charge controller. They reiterated (repeatedly) that the C60 was designed for PV and the warranty would be voided if I used it for anything else ( I knew that). They did say that the C60 can bump the voltage down but not up....... So I wonder if I could possibly set the genset to a higher voltage, say 29v, and let the controller cut that voltage to whatever is necessary to charge the batts - 28+v for bulk and 27v for float, etc.

For longest battery life, your peak charge rate should be c/10 or less, where c is the amp-hour capacity of the battery. Allowing for inefficiencies in charging, this can mean a 15+ hour charge if the battery bank is completely flat. Continuous charging faster than c/5 requires monitoring battery temperature to prevent excessive electrolyte loss.

I wouldn't run my battery bank down to flat - I generally plan to use no more than 60% of the total capacity, and alot of times won't even use that. I don't have my batts in front of me, so don't have the AH capacity available right now, but I think its around 100AH. So with a 4 battery bank, that would be 400AH, figure worst case 60% of that is 240AH, and say I want to charge that in 4 hours, I would need 240/4 = 60 amps, which the genset should be able to deliver. If I got any of this wrong, please let me know.

The charge controller might be fine or it might burn up. It depends on whether it shunts excess source current to ground (shunt regulator) or increases the resistance between the source and load to reduce current (series regulator). A series regulator will be fine. A shunt regulator will fry itself fighting the generator regulator. Good charge controllers have more complicated charging cycles than simple constant voltage charging, and can be programmed to do a lot of clever things like manage rate of charge by bank temperature and adjust cutoff voltage depending on battery bank health and equalization.

Thats what I was trying to find out from the manufacturer - I was hoping that they could/would tell me how it handles excess voltage/current. Also, maybe you can enlighten me - I thought that current would only be delivered as the load needed it, so the genset may be delivering a constant 28v, but it doesn't constantly output the full 70+ amps of current that its capable off...is that right? I know we're getting into more advanced (at least for me) power generation at this point.

Lastly: why are you using 24 volts for the bank? Higher voltages are always more efficient, and cheaper to wire.

I had a few small cheap 24v inverters available, as well as some 24v military lighting, so that's what I ran with - I guess I could wire my 4 12v batteries up and make 48v, but I'd need all new inverters and lights, etc. I understand the efficiencies at higher voltages, but not sure that those efficiencies will actually work out in my favor in my particular use case......maybe you can convince me otherwise.



Thanks again for the input!

 

[steelypip]

Amp hours: You have four batteries in series-parallel to give you 24v. If we take your 100AH rating per 12V battery as true, then this is a 200AH 24V battery. If all four were in series it would be 100AH 48V. All four batteries in parallel gives 400AH 12V. Note that the total power available is exactly the same with all these arrangements, just how it's available varies. Note that 100AH is a BIG battery...

The easiest one to keep equalized is the parallel, as it only has six cells in a series. The hardest would be the 48V string, but it needs the smallest gauge of wire to carry its rated output to a load.

Once you've verified that the Xantrex you're looking at has a series regulator circuit (I suspect they do - regulating a solar cell for longest life means minimizing its current output), then you just crank the generator voltage up to 32V, tell the Xantrex you have a 24V battery, and let it figure out the rest.

 

[PeterD]

I worded that wrong - the fully charged (at rest) voltage of a 12v battery is something like 12.7v, which would put the at rest voltage for 2 fully charged 12v batteries in series at 25.4v, which is lower than the lowest voltage I can set the genset to......so hooking them directly to the genset I would run the danger of overcharging and damaging the batteries unless I stand there with a meter and cut the genset off as soon as they're fully charged....which I don't really want to do ...

You need to understand wet cell charging a bit, but you won't damage the batteries with what amounts to a trickle charge. You can apply 14V to a 12V battery for many hours without any damage at all, and that translates to 28 volts for a 24 volt battery system. That is why those generators are rated at 28 volts!

For a better understanding, monitor both voltage and current. See how much current is flowing into those batteries at 28 volts. Simple math will give you the power (watts) and mass/surface area will show heating achieved. Heat is what damages the battery while charging...

I think you are over-thinking this.

 

[rustystud]

All our busses are 24volt, but the charging voltage is set to 28.4 volts. I know as I have been the "Inspector" which checks all the bus systems for years.

 

[JR2980]

So, I picked up the genset yesterday from the NG armory, and had a few minutes to talk to the generator mechanic that manages all of their gensets - that was a great opportunity.

He pulled the genset off the shelf for me, and gave me all the background on it - only 17 hours on it since new, and he personally drained all of the fluids before it was posted for sale. Said it would fire right up once fluids were replaced, and would run like a top. He wasn't lying - the thing looked brand new, and I took it home, added oil, fuel, bled the fuel lines, and it fired right up! (holy crap is it loud!)

He said that they're getting ready to list another one - he showed it to me....... its brand new, straight from the manufacturer, still in the box with all of the packaging!




As for charging the batteries, I've been talking to several people that have experience here, and so far what I've found is.....no ones really done it this way before, so no one has any experience on what will or will not work. Lots of theory and what it "might" or "should" do, but nothing solid.

First thing I'm going to do is connect an inverter to it, and see if I can get power off of it that way - I have a small 400w 24v inverter that I'm going to try and see if that will work.




Fun times!

 

[Kurt57]

what was the outcome

So, I picked up the genset yesterday from the NG armory, and had a few minutes to talk to the generator mechanic that manages all of their gensets - that was a great opportunity.

He pulled the genset off the shelf for me, and gave me all the background on it - only 17 hours on it since new, and he personally drained all of the fluids before it was posted for sale. Said it would fire right up once fluids were replaced, and would run like a top. He wasn't lying - the thing looked brand new, and I took it home, added oil, fuel, bled the fuel lines, and it fired right up! (holy crap is it loud!)

He said that they're getting ready to list another one - he showed it to me....... its brand new, straight from the manufacturer, still in the box with all of the packaging!




As for charging the batteries, I've been talking to several people that have experience here, and so far what I've found is.....no ones really done it this way before, so no one has any experience on what will or will not work. Lots of theory and what it "might" or "should" do, but nothing solid.

First thing I'm going to do is connect an inverter to it, and see if I can get power off of it that way - I have a small 400w 24v inverter that I'm going to try and see if that will work.




Fun times!

Hi,
I have just installed one of these units in my
off grid home in the Pyrenees Mountains in France. I found this thread while exploring the possibility of connecting up a charge controller to the genset. I was wondering how your experiments worked. Did a solar charge controller handle the generators output OK? I'm finding equalizing the batteries are real pain as I'm constantly monitoring and adjusting the output voltage.

 

[Guyfang]

As long as you guys are looking to buy military gear, why don't you buy the battery charging panel the military has, and then have no fear when charging batteries. Just hook up the DC gen set, hook up the cables to the batteries and rock. Each circuit has its own breaker and is independent of any and all other batteries being charged. Iy my old moss eaten brain can remember what the thing is called in army talk, I will get back to you. They were really nice. Comes in a high speed metal box, so its indestructable. Has cables for 4-6, (?) batteries. Will charge ANY battery. I loved to collect "bad" batteries and "save" 80-90% of them. If anyone here has a SC for the FRESH tool trailer, its in there.