Maximum charge current

lazza · February 2014

HI forum

See attached the design of a system we are about to put in. I used Tinycad- great recommendation from the forum and has given me a great tool to work with.

A have a quick question on maximum charging current:

The 3 Xantrex 60-150 can put out a maximum of 180A combined. The generator will be used to charge too when batteries are low. However I understand that there is a maximum limit of 20% of Ah capacity for charging flooded lead acid batteries (I cant remember if that is at C100, C20 or C10). As the batteries are somewhat undersized for the system at 1500Ah- what current limit setting should I put for the generator charging? Bearing in mind that at certain times both the panels and the generator will be charging the batteries.

Cheers
Larry

Attachment not found.

stephendv · February 2014

Re: Maximum charge current

Check the battery specs and manual, some don't have a current limit below 2.4Vpc. They control this via a temperature limit instead.
The Victron's are completely oblivious to the total current going to the batts, they can only see their own charging current, so if your batts do indeed have a current limit below 2.4V, I think the best way to control it is to set the absorb voltage on the victron's a bit low, e.g. 2.3Vpc. A bit of a "hack", but should work.

Pity you didn't go with the Sunny Island, because it uses a shunt to know how much current goes to the batts from external chargers and will automatically decrease the generator's draw based on what it sees coming from the charge controllers.

vtmaps · February 2014

Re: Maximum charge current

lazza wrote: »

Bearing in mind that at certain times both the panels and the generator will be charging the batteries.

There will not be a problem once you reach absorb voltage, but the currents during bulk charge are additive.

This is something I have ranted about: http://forum.solar-electric.com/showthread.php?17716

An Outback controller, if outfitted with Outback's battery monitor can be programed to throttle back if a particular battery current is exceeded. Midnite has (on their forum) indicated that this feature will be in a forthcoming firmware update to their controller. Stephendv has mentioned the SMA approach. I have no idea what Xantrex controllers do.

Unfortunately Outback throttles back their controller (from solar), not their inverter (from generator).

--vtMaps

BB. · February 2014

Re: Maximum charge current

Around here, we use the 20 Hour discharge rate for our rules of thumbs. With lots of charging current, you will want some sort of temperature monitor to at keep track/alarm if battery bank gets too hot (like it when the charge controller monitors temperature and adjusts charging voltage).

The battery bank is about 1/2 the size we would recommend for the wattage inverters used (200 AH per 1 kW inverter at 24 vdc)... Are these three needed for 3 phase power?

Highly recommend that each inverter DC power goes directly to the battery bus (fuse/breaker per unit). Do not daisy chain from battery bus to inverter to next inverter to final inverter. It makes the required copper wiring (and fusing) three times larger.

Same thing with the MPPT charge controllers, each charge controller has its own home run to the DC battery bus.

And each panel string should have its own series protection fuse/breaker (typically 3 or more panels in series needs a fuse/breaker per its data sheet--Somewhere around 1.25-1.56 times it Isc rating).

Just for a sanity check... If you assume C/8 steady state discharge:

1,500 AH * 24 volts * 0.85 inverter eff * 1/8 discharge rate = 3,825 watts total or ~1,275 Watts per 5kW inverter

Is this what your AC load is (or less)? C/20 is a good average if you assume 5 hours per night for 2 nights with 50% maximum discharge.

C/5 is about the maximum short term load, and C/2.5 is maximum surge (seconds to minutes).

-Bill

Cariboocoot · February 2014

Re: Maximum charge current

The 3 Xantrex 60-150 can put out a maximum of 180A each

What? Since when can a 60 Amp charge controller put out 180 Amps? You mean the three together can put out 180 Amps.

lazza · March 2014

Re: Maximum charge current

BB. wrote: »

Around here, we use the 20 Hour discharge rate for our rules of thumbs. With lots of charging current, you will want some sort of temperature monitor to at keep track/alarm if battery bank gets too hot (like it when the charge controller monitors temperature and adjusts charging voltage).

The xantrex will have temperature sensors

The battery bank is about 1/2 the size we would recommend for the wattage inverters used (200 AH per 1 kW inverter at 24 vdc)... Are these three needed for 3 phase power?

Yes we had to reduce the battery bank at clients request- it means the generator will be functioning more, but it's what they wanted. However, decided to put 3 inverters in, to allow the possibility to switch to three-phase in the future without having to change inverters. The generator can run at 1-ph or 3-ph.

Highly recommend that each inverter DC power goes directly to the battery bus (fuse/breaker per unit). Do not daisy chain from battery bus to inverter to next inverter to final inverter. It makes the required copper wiring (and fusing) three times larger.

Same thing with the MPPT charge controllers, each charge controller has its own home run to the DC battery bus.

Will be doing as suggested- in the drawing I just simplified the connections

And each panel string should have its own series protection fuse/breaker (typically 3 or more panels in series needs a fuse/breaker per its data sheet--Somewhere around 1.25-1.56 times it Isc rating).

I used to do this, but it seemed to always cause problems. Fuses blowing when they shouldnt, extra connections etc.

Just for a sanity check... If you assume C/8 steady state discharge:

1,500 AH * 24 volts * 0.85 inverter eff * 1/8 discharge rate = 3,825 watts total or ~1,275 Watts per 5kW inverter

Is this what your AC load is (or less)?

The steady state discharge should be quite a bit less than that, with moments of peak power up to 10kW.

C/20 is a good average if you assume 5 hours per night for 2 nights with 50% maximum discharge.

C/5 is about the maximum short term load, and C/2.5 is maximum surge (seconds to minutes).

If the batteries cant cope the generator will turn on. The client should have opted for the more expensive option with more batteries, but at least it's something that can simply be added in future (within a reasonable time frame) when he gets mad with the generator![/QUOTE]

Cheers
Larry

Cariboocoot · March 2014

Re: Maximum charge current

lazza wrote: »

Yes we had to reduce the battery bank at clients request- it means the generator will be functioning more, but it's what they wanted. However, decided to put 3 inverters in, to allow the possibility to switch to three-phase in the future without having to change inverters. The generator can run at 1-ph or 3-ph.

The system will not function properly with the too-small battery bank.

I used to do this, but it seemed to always cause problems. Fuses blowing when they shouldnt, extra connections etc.

You must have been doing something wrong. Array fuses/breakers almost never trip, and the extra connections are minimal since they would be in a combiner box.

The steady state discharge should be quite a bit less than that, with moments of peak power up to 10kW.

And there's where the tiny battery bank will suffer most; asked to momentarily produce 10kW. The current draw will be very large and the true battery capacity at that point will drop drastically.

If the batteries cant cope the generator will turn on. The client should have opted for the more expensive option with more batteries, but at least it's something that can simply be added in future (within a reasonable time frame) when he gets mad with the generator!

I never could see the point in putting in solar and then running a generator as a standard part of the system. Generators should be for occasional use only, not a regular contribution to the power supply. It sounds like most of the problems in this system come from the client trying to do being things on a small budget. That, as my grandmother used to say, is inviting the devil to tea.

lazza · March 2014

Re: Maximum charge current

We've had problems with panels here giving more than double their rated amps- I dont know if it's climatic conditions, but 10A fuses were blowing for 5A rated panels. Admittedly these were normal 32V car fuses- sometimes they just melted. But in the end in the smaller systems it was creating more problems than it was solving.

On the combiner box issue. Correct me if i'm wrong, we'd need 3 combiner boxes, 10 breakers or fuses which would increase costs by at least 400€. In fact, if we put in all the recommended midnite stuff, we'd probably increase the costs by double that. The client knocked us down 800€ in the final price simply for fun.

I agree on the generator, i did say it will probably be running every day, and he said that's ok. Some people i'll never understand

. Well it that's what it's about. Making the best of a difficult situation.

Thanks for everyones comments and advice, and will always try to apply recommendations where we can

Cariboocoot · March 2014

Re: Maximum charge current

lazza wrote: »

We've had problems with panels here giving more than double their rated amps- I dont know if it's climatic conditions, but 10A fuses were blowing for 5A rated panels. Admittedly these were normal 32V car fuses- sometimes they just melted. But in the end in the smaller systems it was creating more problems than it was solving.

Okay two things: 32 Volt car fuses are no-go anyway. Just not up to the job. Second the current rating per panel should be either Isc * 1.56 or the maximum series fuse rating given for the panel.

On the combiner box issue. Correct me if i'm wrong, we'd need 3 combiner boxes, 10 breakers or fuses which would increase costs by at least 400€. In fact, if we put in all the recommended midnite stuff, we'd probably increase the costs by double that. The client knocked us down 800€ in the final price simply for fun.

You would need three boxes: one per charge controller. Each box needs to be able to handle the total number of parallel PV connections. I don't know what conditions are like where you are but if you tried that kind of install around here the inspector would not allow it to say the least.

I agree on the generator, i did say it will probably be running every day, and he said that's ok. Some people i'll never understand . Well it that's what it's about. Making the best of a difficult situation.

Thanks for everyones comments and advice, and will always try to apply recommendations where we can

Nothing worse than a client who thinks they know better when they don't. If I ever run across one who does I'll let you know. :roll:

BB. · March 2014

Re: Maximum charge current

We had Wayne from Nova Scotia that had car fuses that about caught fire when operated at 1/2 current.

Stay away from them.

-Bill

lazza · March 2014

Re: Maximum charge current

What fuses would you recommend then? It's very hard to get specialized stuff here.

We tried at first with AC rated fuses for the array and inverter- these simply refused to blow, despite excess current. Have found a source of breakers here now, and we use mega-fuses for the inverters now (the same as the Victron have inside). But the smaller DC fuses are an issue.

BB. · March 2014

Re: Maximum charge current

These are the type of fuses used on this side of the pond with touch safe holders:

http://www.solar-electric.com/mnatm.html

-Bill

Maximum charge current

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