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Can I use a large solar charge controller with a small battery?

Mosesrocket

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There is lots of information on sizing your charge controller to handle the amps put out by your solar panels. You want to make sure your charge controller is large enough to handle the amount of current produced by your panels, as well as be compatable with your battery voltage and type.

But calculators seem to skip on sizing the controller to match the battery capacity or battery acceptance rate. Battery manufacturers recommend that charging current for FLA batteries should be between C/12 amd C/8. So a battery rated at 100Ah should be charged at a rate between 8.3 and 12.5 amps. Apparently, recharging a battery too quickly will shorten its life.

So, if you have a 100 Ah, 12v FLA battery, you should use a solar charge controller rated at about10 amps, and definitely not anything over 15 amps. Is this correct?
 
That would be correct, if no other communication takes place between equipment.
Varies a bit with type and brand. I hear FLA wants 0.12C. My brand of AGM wants at least 0.2C so can charge a bit faster.

Assuming it is an MPPT not PWM controller, you could over-panel with excess PV and multiple panel orientations, so it maintains full current for an extended time during the day.

If SCC can measure battery current, or talk to inverters, then a larger SCC can put extra current on the DC bus when there are loads drawing current, while regulating battery current to optimum 12A or whatever.

A hybrid with PV, battery, AC connections should be able to do this.
Victron charge controllers can communicate with a monitor that has battery shunt, so their output is is adjusted appropriately.
AC coupled systems (like my Sunny Island battery inverter + Sunny Boy PV inverter, and other brands) regulate battery current and command PV inverters to adjust output to match load.


Same issue for lithium batteries. They may take a higher current like 0.5C at comfortable temperatures, so can support a larger PV array. But if operated near freezing, max allowed charge current may be as low as 0.07C or 0.1C
 
Keep 8n mind, during the day, you are charging the batteries AND feeding the inverter loads.
 
I suspect it's a non-issue. In practice an oversized array will typically charge a small lead bank to Absorption voltage (when current acceptance will be dropping) long before the sun is high enough to make serious power. This effect is especially strong with FLA since higher internal resistance means voltage will rise to Vabs faster than other battery types.

Using my campervan system as an example since the data is at hand:

* 220Ah FLA GC2 (C/12 = 18.33A, C/8 = 27.5A)
* 750w solar: normal observed bulk charging max at the shunt is C/13 = 17A, immediately before Vabs is reached mid-morning. The system will make much more power closer to local solar noon but the bank's acceptance rate is already heavily tapered by then.
* Alternator charging (VSR) maxxes at around C/5 = 44A, then levels off around C/10 = 22A
* today is cycle 1,096

> definitely not anything over 15 amps...

<shrug> IMO it's hard to hurt FLA with stiffer charging as long as one uses correct temp-adjusted voltage setpoints, keeps the bank watered, and gets sufficient Absorption duration (see caveat below). They are exceedingly tolerant of abuse; it's not like AGM or gel where aggressive charging can mean immediate/permanent damage.

I want to echo @Hedges observation that charging recommendations vary by manufacturer. Before making any $$ decisions about hardware I'd look up what the battery mfg recommends for the particular FLA.

Caveat: charging lead batteries with firmer Bulk current may require longer Absorption duration, and too many controllers drop to Float prematurely already.
 
Thanks all, good food for thought. Im planning a small RV system, want to put in enough panels and controller for larger loads, but may not have the money for a larger battery bank at this time.
 
The size of the panel dictates the amount of current capable.

The size of the controller needs to be able to handle *at least* the amount your panels can put out - and the rule of thumb back in the day was to double this panel output current value to allow for a controller to easily handle edge-of-cloud lensing affects, which temporarily pushes a panel past its rated value.

Thus, for say an 80 watt nominal-12-volt panel, the max current would typically be (80W / 18v = 4.4 A)

Great, panel is capable of 4.4A, so you'd size your controller to be at least 8.8A, push that up to 10A to make purchase easier to find. Done.

Thing is, the controller is not the one controlling the current, so you could attach this 80 watt panel to a 1KW controller and it would work just fine. Problem is the over-$pending since you didn't need to go that far. :)

But going the other way - putting a 1KW panel into a 10A controller - lets out the magic smoke fast.

Some don't understand how the "CV" setting in a controller works. It is merely an upper limit of voltage that the controller will not allow the system to pass. A nominal 12v solar panel *wants* to charge your battery up to 18v if you let it, but the controller holds the max voltage at the value you set during charge.

What appears to be a tapering of current when in the so-called absorb stage, is NOT a *controller* throttling back current, but the natural physics of when the battery is recharging, the terminal voltages get higher and higher. As the battery-terminal voltages start to get very close to what the controller CV setting is, the difference in voltage between the battery and controller starts to get really small.

It takes voltage to push current. And when the two voltages start to meet up with each other, that *difference* in voltage becomes smaller, and hence is not able to push current as fast as it did earlier on - the so-called taper effect. When the battery-terminal voltage is the same as the CV controller value, there is no difference in voltage (0 volts difference) and then the battery is considered fully charged. (Well a simplification)
 
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The "small" battery sees only the Voltage Difference (between itself and the Solar Controller battery terminals). Many Solar controllers, including even the cheap EpEver "Tracer BN Series", allow you to limit maximum battery current at the Controller as well - in which case, if a big battery bank is happy to accept all the current the SCC is putting out, at a slightly lower Voltage, the output Voltage will not reach the full "Boost Voltage" which your programming allows.

With a large and sunny PV array, and some limiting factor applied within the Solar Controller (for example, it has shifted into "Float", or it has reached it's output current limit), an MPPT controller should generally shift into PWM operation - which leaves "excess" power up in the panels, by rapidly disconnecting and reconnecting the PV array.

As long as your PV does not exceed the rated maximum of your Controller, For either Voltage or total Power (and Power is simply the maximum Volts times the maximum Amps at that Voltage), then the MPPT SCC should be programmed to limit maximum charging amps (when charging a small Lead-Acid battery bank). A "really big" PV Array, requiring a "really big" MPPT Solar Controller, should be tuned to act small - at the Solar Controller.

That consists of wasting money on the "excess" PV and the "excess" MPPT, but it works as a temporary solution (before you upgrade your batteries). It also works for slightly extending the length of the Solar charging Day. In this scenario, with the "big array" and the "big SCC", you might start charging the batteries effectively at 9:45 AM, rather than 10:15. (And still be able to charge the batteries after 2 PM). But you would be wasting most of your Solar power in the peak of the day (maybe 11AM-1PM) by leaving it in the panels, rather than pushing it into batteries.

If you're really concerned about lifespan and you already own the "big" PV array, switch to a longer-lasting type of battery.
 
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