Basic amps question

[Bluedog225]

Good morning,

I’m getting to the stage that I need to build out my solar array and had a really basic question.

Victron system with SOK 48 volt rack batteries.

I’ve got a variety of small-medium MPPT controllers and may get a couple of larger controllers.

I vaguely understand over-paneling and the potential for clipping. And I understand temperature compensated voltage limitations.

What I don’t quite get yet is the significance of putting too many amps on the bus bar that feeds the batteries.

That is, the SOK battery charging limit is 100 amps. What happens if there are three (or more) 50 amp MPPT controllers all putting full power onto the bus bar with a single battery?

Since there are 4 SOK batteries in parallel, does that increase the charge capacity to 400 amps?

But I still have the same question, what if more amps are available?

Do the batteries self limit their input?

What’s the electrical principal here?

Thanks

 

[Q-Dog]

If parallel, add amps together. If in series, adds volts.

So, 4 X 100 ah batteries = 400 ah. And 3 X 50 amp chargers can make 150 amps if all are getting full power.

Victron MPPT can network together, and can limit charge current, so you could limit the 3 50 amp chargers to 100 amps. You would be throwing away potential charging when all are receiving max power.

Be aware that even though lots of batteries list the max charging amps, many also list recommended charging amps. And the recommended charging amps is usually somewhere between .2C and .5C.

 

[rodrick]

The power going in will be distributed between the 4 batteries evenly if wired correctly

 

[SeaGal]

What’s the electrical principal here?

Ohms law

 

[OzSolar]

Since there are 4 SOK batteries in parallel, does that increase the charge capacity to 400 amps?

Depending on you wire them it can but most people parallel them resulting in the first or last battery's terminals see the total system amps. (I think I answered that right)

In that case would refer to the amp rating of the battery terminals.

 

[Bluedog225]

Thanks all. I’m still not getting it.

Assuming the bus bar has the ampacity (I think the Victron lynx is 1000 amps) do I meed to limit the total amps available to the bus bar?

I ask because I can see deploying lots of solar panels. And I’ve got to keep the voltage down per each mppt. What do I do with all the “extra” amps at peak times.

Sorry to be dense.

 

[Q-Dog]

Thanks all. I’m still not getting it.
basic
Assuming the bus bar has the ampacity (I think the Victron lynx is 1000 amps) do I meed to limit the total amps available to the bus bar?

I ask because I can see deploying lots of solar panels. And I’ve got to keep the voltage down. What do I do with all the extra amps at peak times.

Sorry to be dense.

It depends on the bus bar and how you design the system.
This gives basic directions. http://productimageserver.com/literature/brochure/58691BR.pdf

 

[Mattb4]

Good morning,

I’m getting to the stage that I need to build out my solar array and had a really basic question.

Victron system with SOK 48 volt rack batteries.

I’ve got a variety of small-medium MPPT controllers and may get a couple of larger controllers.

I vaguely understand over-paneling and the potential for clipping. And I understand temperature compensated voltage limitations.

What I don’t quite get yet is the significance of putting too many amps on the bus bar that feeds the batteries.

That is, the SOK battery charging limit is 100 amps. What happens if there are three (or more) 50 amp MPPT controllers all putting full power onto the bus bar with a single battery?

Since there are 4 SOK batteries in parallel, does that increase the charge capacity to 400 amps?

But I still have the same question, what if more amps are available?

Do the batteries self limit their input?

What’s the electrical principal here?

Thanks

Let me attempt to answer.

If several SCC are connected to a common DC bus, but only 1 battery, they can output their maximum amps charging so long as the battery is in charge mode and sufficient PV is available. If the total ( SCC1+SCC2+SCC3) is higher than the battery is rated for you can damage the battery.

Batteries in parallel increase the amount you can charge with. Max amps of B1+B2+B3+B4=Max amps charging.

Batteries do attempt to limit themselves but can be damaged by excessive charge rate. SCC are used to provide a proper charge voltage and current. BMS in lithium batteries are there to act as a safety mechanism if the SCC does not work properly (or is set up wrong) and for battery cell balance issues.

The question of what if more amps are available than is proper for maximum battery current? Some form of limit must be used. For instance my AIO's have Max PV charge current and Max total charge current settings. If your SCC's have no charge current settings than you yourself must limit how much PV charging that could happen. This may mean cutting back on panels or Arrays. Or using a smaller amp rated SCC and losing out on total PV production.

 

[mikefitz]

the SOK battery charging limit is 100 amps. What happens if there are three (or more) 50 amp MPPT controllers all putting full power onto the bus bar with a single battery?

The battery BMS will detect over current and reduce the current into the battery. The MPPT chargers cannot produce more current than the battery will accept..

SK48V100 Server Rack Battery Owner & Operator's Manual | Current Connected

Full instruction manual for the SOK SK48V100 server rack battery. Everything you need to know is contained within this document!

www.currentconnected.com

Recomended charge is 63 amps.

 

[Mattb4]

The battery BMS will detect over current and reduce the current into the battery. The MPPT chargers cannot produce more current than the battery will accept..

SK48V100 Server Rack Battery Owner & Operator's Manual | Current Connected

Full instruction manual for the SOK SK48V100 server rack battery. Everything you need to know is contained within this document!

www.currentconnected.com

Recomended charge is 63 amps.

It was my understanding that the BMS simply shuts down charging if it exceeds max. Bit like a overdrawn fuse blows or a circuit breaker trips.

But I am no expert on what BMS's do.

 

[SupraSPL]

The answer to your question is yes. 4 batteries in parallel could take 400 amps in theory, so you could load up the bus bar with (8) 50A MPPTs or any combination of chargers that adds up to 400A.

Of course that thought experiment would describe an out of balance system with 22.4kw of solar and only 22kwh of battery, it would charge in 1 hour of sun. In that system, when the batteries approach full charge and you've got full sun, some batteries will fill up before the others leaving the remaining batteries to take all the charge and maybe trigger the BMS protection, which might be 130 amps if it is a 100 AMP BMS.

Anyway if you added that many panels and chargers, you would want a lot more battery capacity and or a handful of inverters mining crypto or something to put the energy to use.

You mentioned temperature voltage compensation but that is just for lead batteries so no worries there other than keeping them above 32F and below 113F when charging.

 

[Bluedog225]

I was thinking the answer was going to be that there was not a limit to the number of charge controllers you could hook up.

For the easiest example, one 100 amp hour 48 volt battery that wants to be charged at 63 amps. And eight 50 amp mppts on the bus bar.

Perfect conditions, I’ve got 400 amps charging available.

What limits the flow of current to the batteries?

1. The mppt can be set to limit amps. But this would have to limit each MPPT to stay below the total 63 amps.
2. The BMS will only accept what it needs.
3. Isn’t there something to do with increasing voltage bringing down current?

My system is currently powered by 3 panels in series. For discussion, let’s say 9 amps. And the batteries are full. They are not taking in any charge. What is limiting them? Is it the BMS? Or is the MPPT sensing they are full based on voltage?

The MPPT does go from bulk, aborb, float (victron). But I’ve not programmed it to serve the SOK batteries. Maybe I need to?

That is, there is currently available amperage that is not being used when the batteries are full. Wouldn’t that be the same situation if i had 400 amps ready to be delivered?

 

[SupraSPL]

In that example the battery would attempt to take in the 400A and the BMS would trigger over current. If the BMS were NOT in the circuit the cells would take that 400 amps, to the death if necessary.

You could set each MPPT to limit charging current to 12amps, then you would get 100 amp charge current at the battery, but that would defeat the purpose of all the extra MPPTs.

Typically over paneling means putting more panels on an MPPT than it can deliver charging current. That way the MPPT is still serving a good amount of power even when it's NOT sunny. And when it is sunny the MPPT charging current setting limits it to 50A even if there is a lot more solar available, it is unused power.

 

[SupraSPL]

Yes the MPPT voltage charging limit is stopping the MPPT from delivering current in the example you gave.

But you couldn't use the MPPT charging voltage limit current. The MPPT will stop charging the battery once it reaches the voltage limit but the problem there, it could over deliver current before the battery reaches that voltage. For example if you had (8) 50 amp MPPTs on a 100 amp battery, that would still be a no-go because it would try to take that 400 amps on the way up to the voltage limit.

 

[Mattb4]

I was thinking the answer was going to be that there was not a limit to the number of charge controllers you could hook up.

For the easiest example, one 100 amp hour 48 volt battery that wants to be charged at 63 amps. And eight 50 amp mppts on the bus bar.

Perfect conditions, I’ve got 400 amps charging available.

What limits the flow of current to the batteries?

1. The mppt can be set to limit amps. But this would have to limit each MPPT to stay below the total 63 amps.
2. The BMS will only accept what it needs.
3. Isn’t there something to do with increasing voltage bringing down current?

My system is currently powered by 3 panels in series. For discussion, let’s say 9 amps. And the batteries are full. They are not taking in any charge. What is limiting them? Is it the BMS? Or is the MPPT sensing they are full based on voltage?

The MPPT does go from bulk, aborb, float (victron). But I’ve not programmed it to serve the SOK batteries. Maybe I need to?

That is, there is currently available amperage that is not being used when the batteries are full. Wouldn’t that be the same situation if i had 400 amps ready to be delivered?

MPPT is a type of SCC*. Yes the job of the Solar charge controller is to charge your battery. When it reaches the voltage set points on the DC bus it changes from Bulk/Boost to Absorb (CC to CV) and than to float. During Bulk/Boost it attempts to give as much as it can give. If the battery is charged it maintains a certain voltage that the SCC recognizes as being charged and does not output any current. If the battery voltage drops to float it will attempt to maintain float voltage by adding charge.

So yes when you have fully charged batteries, and no other loads, the SCC is sending no current to the DC common bus. My AIO has a charge indicator light (blinking green when charging) turn off. The panels voltage goes up to Voc and nothing is produced.

* More precise it is the logic that a SCC uses to maximize panel output. Maximum Power Point Tracking.

 

[mikefitz]

understanding that the BMS simply shuts down charging if it exceeds max. Bit like a overdrawn fuse blows or a circuit breaker trips.

Read the operators manual, it's not difficult. The management system can limit the charge current if exceeds the 100 amp limit

 

[Bluedog225]

Ok. I better understand over-paneling per MPPT. Makes sense.

To test my understanding:

1. It is possible to over feed my batteries based on the charge controller amp rating.

My four batteries recommended charge is 63 amps each. Therefore 252 amps total. One Victron 450/200 plus a 100/50 and I’m maxed out. The charge controller will reduce output to keep from over charging the batteries over time but it will not keep the batteries from seeing amperage in excess of the battery maximum amperage.

Exceeding the charge controller amp limits would then require the BMS to protect the battery.

2. If I want more panels, and more charge controllers, I need to manually limit the output of the MPPTs to not exceed the battery recommended amps (or battery max amps).

Question: If I limit the output of my charge controllers to stay within battery limits, am I wasting solar power that could be used to power loads? E.g.: air conditioning in the 115 eff summer. I’d have more MPPT amps available but I’d be throttled down to protect the batteries when the air con turned off. The only solution her is to have more batteries?

Question 2: Are the Victron VE.Can controllers smart enough to throttle the whole system back to my preset battery limit? This would be good if I have multiple arrays pointing different directions.

That is, if I have 500 amps of MPPT available, and a 252 amp limit, is the VE.can smart enough to only feed the bus bar 252 amps from whichever source it wants?

I appreciate the help and patience.

 

[Bluedog225]

Read the operators manual, it's not difficult. The management system can limit the charge current if exceeds the 100 amp limit

Yes. Thanks. I’ve gotten the general impression that relying on the BMS is something of a last resort and that it was better practice to use it as a backup. Could be wrong.

 

[SupraSPL]

Point 1 and 2 are correct.

Yes with additional battery capacity you won't be flying so close to the amperage limitations of each battery. When that sun does come out full power you'd have the ability to take it in safely and no waste.

If you just lower the charge current limit on your MPPT charge controllers, I'd imagine that's a more expensive solution than adding more battery and includes power wasted, but that does solve the problem safely.

For question one, Yes if you run an AC unit when there's full sun, to use up some of that excess power instead of letting it get into your batteries. You could do that by putting an inverter on the same bus bar as the batteries and the energy won't even bother going into the batteries it'll go directly to the inverter.

 

[rodrick]

Why is the charge rate .63c on these batteries normally .5c is what I thought was common and for longer life .2 to .3c was better?