Need advice on offline solar charge controller - which configuration, and is the power adequate?

Thread Starter

Bhante

Joined Dec 15, 2009
64
I need to use solar panels with lead-acid batteries and 230V inverter. There are two putative connection configurations in my mind, with different consequences/requirements.

Components:
1) Solar panels, 2 in series, 412W, 40V at full power
**2) Putative charge controller Suoer ST-MP40A (can take PV up to 150V, rated 40A, with 40A DC output for DC load). The manual gives no specifications of the "DC output".
3) 2x 12V car batteries 100Ahr each, wired in parallel [yes I know, that doubles the current, but the inverter is 12V so I have to use 12V]
4) Inverter Suoer FPC-2000A (12VDC to 230VAC) rated for continuous 2000W peak 3000W in theory (I have doubts about that, but maybe so if the battery is humungus; at any rate it does provide 1500W as long as the battery is sufficiently well charged to remain above 10V under load). Inverter cut-off voltage is 10V under load.
5) Load 1450W resistive load (espresso machine). At the inverter cutoff of 10V, this is drawing 145A from the battery. It will need both battery and solar simultaneously to produce enough power.

** This item is the crux of the question: according to the configuration, either 40A is OK, or I need 150A rating if inverter is wired through controller.

1) First putative configuration: Ignore the "DC output", connect solar cells to controller, controller to battery, and inverter directly to battery without going through controller.
In this case the controller rating at 40A is adequate to handle output from up to 3 solar panels at full output, and the inverter current drawn from the battery is irrelevant. However my doubt is whether the charge controller needs to be able to measure load current in order to regulate the charge correctly? If I bypass the DC output, my fear is it will overcharge the battery and quickly destroy it, because by drawing current the inverter heavily depresses voltage.

2) Second putative configuration: Connect the inverter through the "DC output" of the charge controller. In this case the controller can monitor the load, and might be better equipped to charge the battery correctly, but I would then need a far bigger charge controller.

(1) or (2)?
 
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nsaspook

Joined Aug 27, 2009
10,683
Please, don't use a 12 volt system, the copper losses will be excessive unless you use very large wiring that likely won't fit the CC terminals correctly.. Go with and design for 24 or 48 volts (higher voltage -> smaller wire gauge for the equivalent power carrying capability) for the battery bank, inverter, with the MPPT charge controller. For 24 volts simply put the batteries in series or add two more for a total of 4 series batteries for 48vdc.

Use the CC correctly to charge the battery and to supply power the loads with the battery bank. they will work together to supply power to the load. Don't direct connect the panels to the battery.

A typical 24v battery bank/inverter system.
https://forum.allaboutcircuits.com/...c-controlled-battery-array.32879/post-1488959
 
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Irving

Joined Jan 30, 2016
3,190
1450W output from inverter = 1700W input at 85% efficiency (typical), = 170A @ 10v except battery wont get close to 10v at that current. 80Ah * 2 = 160Ah =20h @ 8A but at 120A initially that's only about 10 - 15min runtime due to Peukert.

The maximum load on the DC output for that controller is 25A.

So direct connect to the inverter is your only option.

But i wouldn't worry about the charger damaging the battery, the inverter pulling that load will do it for you...

12v is totally impractical and you need a lot more panels to get remotely viable.
 

Ian0

Joined Aug 7, 2020
6,703
And don't use a "car" (SLI = Starting Lighting and Ignition) or you will be replacing it so often that primary cells look like a viable option. You would be lucky to get 100 full charge-discharge cycles from a SLI battery. A "Leisure" battery would give you 300 cycles.
Get yourself some tubular plate batteries either ex-Fork-lift or from a manufacturer such as Rolls or Trojan. Expensive, but will save money in the long run, and still cheaper than lithium.
 

nsaspook

Joined Aug 27, 2009
10,683
12v is totally impractical and you need a lot more panels to get remotely viable.
+1

I built a 12vdc system that can actually handle 1800W for a decent amount of time. The reduction of voltage drops and current capacity requirements at 12vdc are a mess that require 0000 gauge wire to minimize losses.
https://forum.allaboutcircuits.com/...c-controlled-battery-array.32879/post-1438996
1669483125281.png
https://www.renogy.com/deep-cycle-agm-battery-12-volt-200ah/

Wish it was 24vdc like the next system I designed and built.
 

nsaspook

Joined Aug 27, 2009
10,683
Yeah, but you had 800Ah+ to play with... still would have been better at 48v!
I can recommend that brand of Deep Cycle AGM if you want to stick to Lead-acid chem.
1669484408444.png
Maximum 2000A discharge (5 seconds) can handle most motor starting surges if the Inverter can also handle it.
 

nsaspook

Joined Aug 27, 2009
10,683
No thanks, I'm a LiFePO4 afficionado!
Those are great too but still pricey at my system current Ah PNW requirements that are mainly for summer AC runs and winter emergency power. I try to keep the 50% battery cycles low to extend life-time. After 3 years, the AGM set is still running strong.
LiFePO4 will be in the running when I finally move south (central Texas) after retirement. It's a possible remote location off-grid so battery cycles and energy requirements seem to match the LiFePO4 profile better than AGM.
 

Thread Starter

Bhante

Joined Dec 15, 2009
64
[I've corrected the original post, batteries are actually 100Ahr not 80Ahr]

Many thanks to Irving and nsaspook for your various helpful replies. I agree about the unsuitability of 12V - but unfortunately I am stuck with it. (I do have an old 24V inverter, but doesn't seem to be working any more, and I can't get any circuit diagrams for it nor competent service.) Inverters above 1000W around here are very hard to find, highly expensive, and usually crap.

But i wouldn't worry about the charger damaging the battery, the inverter pulling that load will do it for you...
Well, an ordinary car battery is designed to draw up to several hundred amps for starting the motor! But this is admitedly for longer, that's probably what you are referring to?

I have to say I've been really shocked at the short lifetime of batteries over the last 8 years, but mostly that seems to have been for other reasons. I first had two smaller batteries about 60Ahr I think which lasted nearly 2 years with a 24V inverter. After that the inverter had an overcharging problem and chewed up batteries really fast. The next batteries were two sealed batteries which lasted only 3 months - I said never again to sealed batteries (impossible to top up water)! But with subsequent batteries I never got much more than a year. In part I blame putative bad quality locally produced acid and top-up water. The local battery water has very high TDS. Batteries are also extremely expensive here - that is, normal car batteries, forget about specialised batteries! At one time I had two rather expensive Panasonic N100PI batteries "for inverter" - but actually got much better performance out of the 60Ahr car batteries (despite being the 60Ahr ones being slightly imbalanced from the outset) - probably because the inverter was charging badly by that time. The last two years I had to switch to the 12V system, hence I am stuck with 12V for now. I will move next month, thereafter only solar, completely off-grid. I am reckoning the coffee machine will only be useable with both solar and batteries in conjunction.

In theory, is the charge controller designed only to have a DC load that passes through it, or can it cope with monitoring charge reasonably well when the load is connected directly to the battery?

@Irving and @nsaspook

What do you think, should I go with the ST-MP40A or avoid it? If I stick to it we need to pay the deposit before the end of the month. (And if not ... well, that's another problem, possibly!)
 
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Thread Starter

Bhante

Joined Dec 15, 2009
64
For your information, an auto SLA battery is considered dead when it’s voltage is only 10V.
So, according to this wonderful theory of yours, when a "dead" battery that is in the middle of delivering, say, 150A under load has a momentary voltage across its terminals of 10V, what exactly is "dead" about it? And when the load is removed and the voltage across the terminals recovers to over 12V, has the battery now come back to life, or is it still "dead" according to your fascinating definition?
 

Irving

Joined Jan 30, 2016
3,190
So, according to this wonderful theory of yours, when a "dead" battery that is in the middle of delivering, say, 150A under load has a momentary voltage across its terminals of 10V, what exactly is "dead" about it?
10v = >90% Discharged. At that point the internal resistance due to sulphation of the plates starts to rise rapidly and pulling high currents generate heat and the mesh can start to melt at hotspots as well as hardening the sulphation to be non-reversible... both of these issues are terminal for the battery if continued. It may not seem that way, but its reducing the cells ability to charge and can lead to premature cell failure. Just because you can draw 150A @ 10v doesn't mean you should!

voltage across the terminals recovers to over 12V, has the battery now come back to life
No it hasn't 'come back to life'. Voltage measured open circuit is always around 2.2v per cell unless the cell is chemically inert (and therefore unusable). As soon as you start drawing current again the terminal voltage will drop due to the internal resistance - that hasn't gone away, though over the course of 24h or so it may improve a bit but it will return rapidly as soon as you put any load on the depleted cells. Only a recharge and soak for 12 - 16h will reverse the situation and truely bring it back to life (and if its been stressed, slightly less life than before!).
 

Thread Starter

Bhante

Joined Dec 15, 2009
64
10v = >90% Discharged.
Wow! A lot of misconceptions here. If it's 10V open circuit then yes, it is 90% discharged. I'm not talking about that. I am talking about 10V <i>momentary voltage</i> while under 150A load, followed by recovery - on removing the load - to a much higher open-circuit load. That isn't possible with a battery that is 90% discharged. If you try that with a 90% discharged battery then as soon as you connect the load the terminal voltage will drop very low (5 - 6 - 7 volts, say), the current drawn will be far less, and the recovery will be to something probably not that much above whatever it dropped to under load. Two totally different scenarios.

Action and reaction - when you draw current, the voltage immediately changes, it does not stay constant.

Voltage measured open circuit is always around 2.2v per cell
I suggest you try measuring the open-cell voltage of a few batteries under different states of charge - you'll find you get very different results! Open circuit voltage - while not more than a very crude and inadequate measure of state of charge - is nevertheless highly dependent on state of charge.

You could start by measuring the voltage of your car under various conditions, before and after a trip of say, 20-40km. First measure the open circuit voltage <i>before</i> the ignition has been switched on at all that day. Then start the motor, imediately switch off the engine after starting, then measure again open circuit voltage. Assuming the battery was not fully charged the change will be fairly small, but there should be a measurable drop due to drawing current for the starter motor, which draws a very high current compared to current while driving. Then immediately after driving some 20-40km at a decent speed, again measure the open circuit voltage - it will be higher, due to recharging by the alternator (as long as it was not all 'start-and-stop' driving in traffic, of course, where the charging by the alternator is very small).
 
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nsaspook

Joined Aug 27, 2009
10,683
A 12V nominal battery bank dropping to 10V under load is either a bad design (undersized or wrong type cells) or the batteries are shot/discharged.
 

Irving

Joined Jan 30, 2016
3,190
Wow! A lot of misconceptions here. If it's 10V open circuit then yes, it is 90% discharged. I'm not talking about that. I am talking about 10V <i>momentary voltage</i> while under 150A load, followed by recovery - on removing the load - to a much higher open-circuit load. That isn't possible with a battery that is 90% discharged. If you try that with a 90% discharged battery then as soon as you connect the load the terminal voltage will drop very low (5 - 6 - 7 volts, say), the current drawn will be far less, and the recovery will be to something probably not that much above whatever it dropped to under load. Two totally different scenarios.

Action and reaction - when you draw current, the voltage immediately changes, it does not stay constant.
I'm not disagreeing in principle. According to MK, the OC voltage of a completely depleted SLA is 11.7v and on 0.1C (8A for an 80Ah) should not be less than 10.8v
if your non-depleted battery is dropping to 10v on a 150A load then its internal resistance is much higher than I would expect. A good SLA has an internal resistance of 8 - 10mohm so at 150A I'd expect to see at least 11.5v.

As a permanent electric wheelchair user, batteries are my lifeline - I used to go through deep-discharge 73Ah SLAs at the rate of a pair every 10 - 12months - and that was the good ones (MK, Odyssey), a typical cheap Chinese one lasts 5-6 months on average. A powerful chair with 2 x 450W motors on soft off-road surfaces can regularly pull 120A, and I can discharge a set in less than 1/2 a day (and under 3 miles!), needing a 12 - 16h recharge on a 10A charger.. So I think I know something about how these things behave. Starting a car is trivial by comparison. An 80A battery at continuous high discharge rates has a real capacity of 50-60Ah, you simply cannot use the last 20 - 30Ah because the volts drop too low to be useful, and I mean <11v!

Nowadays I run 210Ah LiFePO4 in the same physical space as an 80Ah SLA and I can truly use 95%+ of its capacity (and then recharge, if I need to, in under 5h at 50A, though I tend to use 25A to be kinder). My current pack is 2y old and shows no signs of degradation. Recent developments I am aware of suggest that soon I can expect to get closer to 300Ah in the same space!
 

Thread Starter

Bhante

Joined Dec 15, 2009
64
Did you ever try comparing sealed with flooded, and if so how did they compare and how much did you have to top up the flooded?
 
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