48v DC Generator to 48v Battery Bank

[YellowBuddy]

My familiarity is with AC generators as DC generators seem pretty rare to start with, but I just found some surplus ones that put out 48v DC. It just happens to match my battery bank perfectly; which is feeding the house via SMA Sunny Islands.

Before I jump on the purchase, I just wanted to confirm as there seems to be conflicting information. My plan is to utilize the Sunny Islands to control the generator - on/off and prevent overcharging. Since they're the same voltage as the battery bank, I don't need any step up or step downs. Do I need anything to control that charge rate?

Is there anything I'd need specifically? Or is it really as simple as - hook it to the battery to replenish the bank?

Thank you,

 

[JoeHam]

You will need something greater than 48v to charge 48v nominal batteries.

Probably in the mid 50’s

 

[YellowBuddy]

You will need something greater than 48v to charge 48v nominal batteries.

Probably in the mid 50’s

Should have clarified. It’s voltage range is configurable from 48vdc to 54vdc. My sunny island seems to put our 48V on float and about 53V in charge boost. So the upper end of that should work right?

 

[Ampster]

Should have clarified. It’s voltage range is configurable from 48vdc to 54vdc. My sunny island seems to put our 48V on float and about 53V in charge boost. So the upper end of that should work right?

I think Boost is another term for Absorb or Constant Voltage and if that is the correct voltage for your batteries that should work. Do you have any control over Amperage?

 

[Ampster]

I see from your other thread that those generators put out 150 Amps.

 

[John Frum]

My familiarity is with AC generators as DC generators seem pretty rare to start with, but I just found some surplus ones that put out 48v DC. It just happens to match my battery bank perfectly; which is feeding the house via SMA Sunny Islands.

Before I jump on the purchase, I just wanted to confirm as there seems to be conflicting information. My plan is to utilize the Sunny Islands to control the generator - on/off and prevent overcharging. Since they're the same voltage as the battery bank, I don't need any step up or step downs. Do I need anything to control that charge rate?

Is there anything I'd need specifically? Or is it really as simple as - hook it to the battery to replenish the bank?

Thank you,

What chemistry are your cells?

 

[YellowBuddy]

I see from your other thread that those generators put out 150 Amps.

Correct, max of 150A. It has an onboard controller that can shut off based on SOC but I don’t see any evidence of amperage control, only voltage control.

It’s a smaller version of this:

http://filecenter.deltaww.com/Products/Download/09/Fact%20sheet_PowerGen%2010000_en_rev02%20(W).pdf

What chemistry are your cells?

Lead acid AGM. They’ll be replaced within the next two years. They’re about 7 years old and I need to expand the bank. So longevity isn’t too much of a concern for this set, but will be for the next set.

 

[Ampster]

Then the more important question is what is the capacity. AGMs can't tolerate Amps like Lithium batteries but you would have to figure out how that applies to your specific pack.

 

[Hedges]

It out to put out less if run slower. Less current and/or voltage, but may or may not maintain a reasonable current over the whole voltage range of your battery (depending on DoD)

Here's something you're going to have to deal with:

Output Voltage-52 VDC(PositiveGround, Configurable -48 to -54VDC)

Ordinarily SI battery negative would be grounded.

 

[YellowBuddy]

Then the more important question is what is the capacity. AGMs can't tolerate Amps like Lithium batteries but you would have to figure out how that applies to your specific pack.

Current is only 48v/100ah or 4.8kwh and I think the generator would be overkill. Future is targeting to be 48v/1520ah or just about 73kwh.

Here's something you're going to have to deal with:

Output Voltage-52 VDC(PositiveGround, Configurable -48 to -54VDC)

Ordinarily SI battery negative would be grounded.

I didn't give that any thought whatsoever until you posted that. Previously, as far as I knew ground was ground. I'm trying to process what that would mean and google is giving some conflicting information.

The car folks seem to think, you "just turn the battery around"
Then there's articles saying "positive is still positive" but it's "just no longer hot"

I'll admit, I'm scratching my head here...

 

[Ampster]

That is way too much Amperage for a 100 Ahr AGM. My ballpark guess is 0.1C which would be 10 Amps. AGMs might be able to take a little more.. That would work for that larger pack or a smaller Lithium pack.
My battery is not grounded but my inverter is so I don't know for sure. There was a thread here I believe about old Sunpower panels which were positive ground. There was a work around of some sort.

 

[YellowBuddy]

That is way too much Amperage for a 100 Ahr AGM. My ballpark guess is 0.1C which would be 10 Amps. AGMs might be able to take a little more.. That would work for that larger pack or a smaller Lithium pack.
My battery is not grounded but my inverter is so I don't know for sure. There was a thread here I believe about old Sunpower panels which were positive ground. There was a work around of some sort.

Indeed. It’a more for the future pack. At 1520, 10% would be 152 amps. Which would make this kind of perfect in terms of size.

With the current pack, it would end up using it only if I had sufficient draw as well.

My batteries aren’t externally grounded as far as I can see. The cabinet is grounded. My inverter uses L1, L2, and G. I’ll see if I can dig up that old post.

 

[Hedges]

Problem is if positive side of the generator is tied to chassis and chassis is grounded to gas pipes etc, you now have -48V. Sunny Island would normally have negative side of battery grounded and you want to charge the positive which is +48V.

There might be nothing wrong with grounding battery positive and putting fuse on negative. Then again, could be some issues.
I think cars switched from positive ground to negative to reduce corrosion. May not be such an issue for Sunny Island.
Ethernet/Speedwire should be transformer isolated. RS-485 is going to be ground referenced, or rather now -42 to -54V referenced. Is that being used, and will other end care?

Best if you could rearrange the generator, but don't know what it's circuitry looks like. If it was just rectifiers with one terminal grounded it would be easy.

"That is way too much Amperage for a 100 Ahr AGM"
But perfect for the 1520ah he thinks he's going to get, if really wants to spend $20k.
With a small battery, if an analog current control of the generator was available, a servo circuit regulating current measured in a shunt would be good.
Then the generator could power house loads through Sunny Island plus desired charge current.
Maybe just a governor circuit to control engine speed.

 

[YellowBuddy]

But perfect for the 1520ah he thinks he's going to get, if really wants to spend $20k.

Im not willing to drop $20k on the battery. But luckily, I know of a company that’s migrating their data center and will be dumping their equipment. They’ve got some age on them, but they’ve haven't been cycled much and well maintained and 1/3 of the cost.

Ok so it sounds like I need to take the following precautions:
1. Install a separate ground rod and ground the generator separately (I believe it’s required by code anyway)
2. Explore circuits.

If reconfiguring, is it a bonding type thing where the positive or negative terminal is actually connected to ground?

 

[Hedges]

I was going to say I thought of a problem with positive ground, but looks like you're in luck:

"6.4.7 BatVtgOut Power Supply
The battery voltage is conducted to the outside at these terminals. The battery voltage is fused at both
poles by PTC resistors (max. 0.75 A). Depending on the internal temperature of the Sunny Island, the
tripping threshold is over 0.75 A.
This connection can be used, for example, to supply a DC contactor for load shedding."

If Sunny Island had only fused the +48V "BatVtgOut" + terminal and not the - terminal, grounding battery positive would have meant a short of the BatVtgOut - terminal in relay circuits would have been unprotected, but they did fuse both.

Battery shunt, which you'll need for DC charging, has to be on battery negative terminal. Manual says to use intrinsically safe wire, meaning it acts as a fuse. Of course you could put fuses or PTC at the shunt.

I wonder about RS-485 and Sync cables, if they have a negative wire that could be a problem shorting to chassis.

You should contact SMA service, see if grounding positive battery is acceptable.

Unless you can rewire the generator for negative ground. Yes, I would think it does have chassis tied to positive.
Looks like it has a rackmount box of electronics in addition to the generator. Probably the generator itself is isolated (or would be if you rewired its rectifier). You could disconnect the electronics, isolate the rectifier, and just run generator until battery reaches desired voltage. Electronics may control current in a rotating winding (like an automotive alternator) so you would need to drive/regulate that. It says 9-phase alternator; you could put 3 or more phases through very small isolation transformers and rectify that to feed the generator's electronics - that will regulate voltage pretty well, but isolated from the the main output.

 

[YellowBuddy]

I was going to say I thought of a problem with positive ground, but looks like you're in luck:

"6.4.7 BatVtgOut Power Supply
The battery voltage is conducted to the outside at these terminals. The battery voltage is fused at both
poles by PTC resistors (max. 0.75 A). Depending on the internal temperature of the Sunny Island, the
tripping threshold is over 0.75 A.
This connection can be used, for example, to supply a DC contactor for load shedding."

If Sunny Island had only fused the +48V "BatVtgOut" + terminal and not the - terminal, grounding battery positive would have meant a short of the BatVtgOut - terminal in relay circuits would have been unprotected, but they did fuse both.

Battery shunt, which you'll need for DC charging, has to be on battery negative terminal. Manual says to use intrinsically safe wire, meaning it acts as a fuse. Of course you could put fuses or PTC at the shunt.

I wonder about RS-485 and Sync cables, if they have a negative wire that could be a problem shorting to chassis.

You should contact SMA service, see if grounding positive battery is acceptable.

Unless you can rewire the generator for negative ground. Yes, I would think it does have chassis tied to positive.
Looks like it has a rackmount box of electronics in addition to the generator. Probably the generator itself is isolated (or would be if you rewired its rectifier). You could disconnect the electronics, isolate the rectifier, and just run generator until battery reaches desired voltage. Electronics may control current in a rotating winding (like an automotive alternator) so you would need to drive/regulate that. It says 9-phase alternator; you could put 3 or more phases through very small isolation transformers and rectify that to feed the generator's electronics - that will regulate voltage pretty well, but isolated from the the main output.

I just gave them a call, they didn't have much experience with positive ground nor 48V dc charging. With limited confidence the guy told me - it won't affect the SI.

To get more confirmation, I also found this:

Connecting the Grounding Conductor with a Grounded Battery

manuals.sma.de

"If you ground the battery, you can ground it at the positive terminal or at the negative terminal with a grounding conductor. SMA Solar Technology AG does not recommend grounding the battery. If the battery is grounded, the enclosure of the Sunny Island must also be grounded."

So based on that, it doesn't appear to matter if the battery is positively or negatively grounded - or grounded at all. Now I'm wondering if the batteries will charge off the positive ground if I don't ground the batteries.

 

[Hedges]

10 kW generator consumes NG : 8.88 lbs/hr at full load

100,000 BTU/therm
20,000 BTU/pound

My rate (at least for low usage) was $1.30/therm last December

Would cost me $1.78 in gas to run for an hour, or $0.178 per kWh.
Not bad in my location ... utility rates are $0.15, $0.30, $0.45/kWh depending on time of day.
I think elsewhere in the country some people pay as little as $0.05/kWh

Figure on maintenance every 50 operating hours, every 500 kWh. Oil and filter about $20, maybe $20 of your time? Another $0.08/kWh.
$0.26/kWh total, assuming you get the generator for cheap.
I much prefer PV at $0.05/kWh but of course you would be willing to pay much more to be comfortable during storms.

 

[Hedges]

I just gave them a call, they didn't have much experience with positive ground nor 48V dc charging. With limited confidence the guy told me - it won't affect the SI.

To get more confirmation, I also found this:

Connecting the Grounding Conductor with a Grounded Battery

manuals.sma.de

"If you ground the battery, you can ground it at the positive terminal or at the negative terminal with a grounding conductor. SMA Solar Technology AG does not recommend grounding the battery. If the battery is grounded, the enclosure of the Sunny Island must also be grounded."

So based on that, it doesn't appear to matter if the battery is positively or negatively grounded - or grounded at all. Now I'm wondering if the batteries will charge off the positive ground if I don't ground the batteries.

Manual says,

"External grounding of the negative terminal of the battery
External grounding of the negative terminal of the batteries is possible because the batteries and
the grid side are galvanically insulated within the Sunny Island.
• Dimension the cross-section of the grounding conductor sufficiently. Thus, you are ensuring
that in the event of a fault the high currents occurring can be discharged with an external
grounding.
• If grounding of the negative terminal of the battery is necessary, assemble this outside of
the Sunny Island."

So that indicates grounding is optional. At about 54V ... 60V during any equalizing (default for AGM appears to be "equalize" every 6 months but with voltage no higher than absorption so doesn't really happen). That may be just above the voltage considered safe for people when exposed. If you're soaking wet it might be a problem.

If you don't ground at SI, the generator will ground positive through its chassis. Charging will happen because you route positive and negative cables from generator to battery. You ought to have a fuse on negative at battery, and run a ground wire from SI to generator sufficient to blow that fuse in event negative cable shorts to SI case at the feed-through or anywhere else. That is so fault current doesn't run through earth ground or skinny wires like AC ground wires or through the gas pipe. It could melt the flexible gas line, providing fuel and ignition source.

Or I think you could isolate the generator electromagnetics from electronics with transformers as I proposed earlier. But simply doing positive ground seems simplest.

 

[snoobler]

Im not willing to drop $20k on the battery. But luckily, I know of a company that’s migrating their data center and will be dumping their equipment. They’ve got some age on them, but they’ve haven't been cycled much and well maintained and 1/3 of the cost.

If you're talking about batteries from backup or UPS-type systems, those batteries typically do NOT handle cycles well and deteriorate rapidly even with light cycling. They are mostly a starter battery designed to be floated most of their lives, handle high current output and increased charge current with only marginally improved cycling capabilities over a traditional automotive starter battery. When you consider the application, you don't need cycle life as you hope to use them as little as possible.

The price may be right, but the long term value may be very poor.

 

[YellowBuddy]

10 kW generator consumes NG : 8.88 lbs/hr at full load

100,000 BTU/therm
20,000 BTU/pound

My rate (at least for low usage) was $1.30/therm last December

Would cost me $1.78 in gas to run for an hour, or $0.178 per kWh.
Not bad in my location ... utility rates are $0.15, $0.30, $0.45/kWh depending on time of day.
I think elsewhere in the country some people pay as little as $0.05/kWh

Figure on maintenance every 50 operating hours, every 500 kWh. Oil and filter about $20, maybe $20 of your time? Another $0.08/kWh.
$0.26/kWh total, assuming you get the generator for cheap.
I much prefer PV at $0.05/kWh but of course you would be willing to pay much more to be comfortable during storms.

That's my situation as well. Utility rates are an average of $0.157/kwh for me. So at that price it isn't a shabby alternative during extreme situations. My primary is always going to be my solar/grid first; which had an immediate breakeven. My secondary/insurance is going to be solar/off grid battery. The generator would be the insurance policy to my insurance policy.

Manual says,

"External grounding of the negative terminal of the battery
External grounding of the negative terminal of the batteries is possible because the batteries and
the grid side are galvanically insulated within the Sunny Island.
• Dimension the cross-section of the grounding conductor sufficiently. Thus, you are ensuring
that in the event of a fault the high currents occurring can be discharged with an external
grounding.
• If grounding of the negative terminal of the battery is necessary, assemble this outside of
the Sunny Island."

So that indicates grounding is optional. At about 54V ... 60V during any equalizing (default for AGM appears to be "equalize" every 6 months but with voltage no higher than absorption so doesn't really happen). That may be just above the voltage considered safe for people when exposed. If you're soaking wet it might be a problem.

If you don't ground at SI, the generator will ground positive through its chassis. Charging will happen because you route positive and negative cables from generator to battery. You ought to have a fuse on negative at battery, and run a ground wire from SI to generator sufficient to blow that fuse in event negative cable shorts to SI case at the feed-through or anywhere else. That is so fault current doesn't run through earth ground or skinny wires like AC ground wires or through the gas pipe. It could melt the flexible gas line, providing fuel and ignition source.

Or I think you could isolate the generator electromagnetics from electronics with transformers as I proposed earlier. But simply doing positive ground seems simplest.

They followed up with emails on the manual, including instructions on the shunt calibration to make sure the SI reads correctly. I haven't heard back from the generator manufacture yet but I reached out to them as well about switching the ground. I'm pretty sure it could be done as well. But it's good to know that I won't be spending money for a brick here and it'll be usable either way.

If you're talking about batteries from backup or UPS-type systems, those batteries typically do NOT handle cycles well and deteriorate rapidly even with light cycling. They are mostly a starter battery designed to be floated most of their lives, handle high current output and increased charge current with only marginally improved cycling capabilities over a traditional automotive starter battery. When you consider the application, you don't need cycle life as you hope to use them as little as possible.

The price may be right, but the long term value may be very poor.

They're rated for 2400 cycles at 20% DOD. So you're completely right here. But I'm also not using them in a full off grid situation and will be using them very much like a data center. They'll sit in float most of their lives. In an outage, I'll hopefully be running off of my solar. At night (or cloudy) I'll be switching to battery and I still have the generator to top off as well. It certainly has a narrow usability range, but the price is really right in the short term. I can't afford what I want to do in the long term just yet..maybe another 5 years.