Solar charger - tempature compensation really needed?

[marty9876]

I'm building out a 200w solar setup (might expand to 300w) and I'm stuck on controller selection. MPPT seems to be a given, I want to stick with the popular brands. While I read tempature compensation has its value, I'm just trying to determine how much value it really is. At this size of controller, only one of the controllers actually comes with the remote tempature sensor the rest is an add on which leads me to think it might not be too important.

Basically, is it worth $150-$200 to add tempature compensation?

I'm trying to get myself settled on a Victron BlueSolar 75/15 MPPT (I know it would only do 200w and has ambient air tempature compensation- not on battery). With the blue tooth dongle it's $150, compared to a Morningstar for $300-$350.

Looking at Victron, Morningstar, Blue Sky, Midnite, Outback, Western.

[a64pilot]

I paid about $20 for the one for my Outback, you sure about that price?

[marty9876]

Originally Posted by a64pilot

I paid about $20 for the one for my Outback, you sure about that price?

I might not have been clear- not the cost of the cable it's the cost of the controllers which leverage tempature compensation.

That said, for a Outback SCCM20-100 Amazon want $299 but everyplace else online is ~ $150 for that controller. Amazon pricing was messing me up I think- off to read another controller manual!

[StuM]

It's often said that temperature compensated charging can add up to 15% of battery life. But that is if the batteries are subjected to very large temperature variants.

So if your batteries are going to see very large temperature variations AND you have a large, expensive battery bank, it may be worth it. Otherwise, I wouldn't bother.

If you've got a couple of hundred bucks worth of batteries, even a 15% increase in life is only worth $30 every few years. $300 extra on your charger will take 10 sets of battery replacement to pay for itself.

(ignoring all those funny accounting things like discount rates and present/future value)

[john61ct]

Absolutely 110% yes, not just voltage compensation but scaling current down if batt gets too hot.

Also variable voltage setpoints, not just a canned batt type list but actual voltage level desired, absorb and float separately in .1V increments.

Ideally, also customize how long Absorb runs before going to float.

Above for any charging source, not just solar.

Victron, Blue Sky, Midnight or Morningstar

[txg]

votronic also has mppts with good features and external temperature sensors for a good price

[MarkJ]

If you turn it down a bit in summer and turn it up a bit in winter you save the money and another bit of corrodable metal on a battery terminal.

[a64pilot]

Another way if you don't mind the hassle is a simple remote thermometer, read the temp and change the voltage yourself.
Batteries are of course a huge thermal mass and therefore quite resistant to temperature changes, the temp difference in the hottest part of the day to the coolest part of the night is negligible.
They most variation I think I have ever seen my charging voltage is plus or minus .1 V.

However my bank is in the boat, specifically under the settee. Now if your bank is in the engine compt., then of course that is different, a long motor would likely raise the temp quite a lot.

[basssears]

Thanks for starting this thread, I've had the same question and look forward to reading the answers.

Originally Posted by marty9876

I'm trying to get myself settled on a Victron BlueSolar 75/15 MPPT (I know it would only do 200w and has ambient air tempature compensation- not on battery).

Just to clarify on this controller (because I am leaning towards this one too so have been researching it), its upper limit is based on amperage not wattage. So you're not limited to 220w on this controller, you're limited to 220w at 12V... you can run up to 440w into this controller if you go 24v. I'm going to start with 200w @ 24v into this controller, knowing if I want I can add another 200w as long as I keep that at 24v as well.

Not been able to clarify why one would pay the extra $40 for the Victron MPPT 100/15, the difference between accepting 75V and 100V open doesn't seem to make much difference to me at least, don't ever envision going above 24v but even so 48v is still well within the 75/15.

[marty9876]

Regarding the Victron controller, this has me pretty confused from the manual:

3.2. PV configuration
The controller will operate only if the PV voltage exceeds battery voltage (Vbat).
● PV voltage must exceed Vbat + 5V for the controller to start. Thereafter minimum PV voltage is Vbat + 1V.
● Maximum open circuit PV voltage: 75V.
The controller can be used with any PV configuration that satisfies the three above mentioned conditions.

The 'start' part where PV voltage must be +5v from a Vbat. Not sure what start means, in my particular setup I'll have 2x 100w panels @ 17.7v. Current thinking is to stay parallel - conservative approach plus I may add 2x 50w panels and mixing those in series would greatly reduce the output from my understanding (mixed watt panel same volts scale linearly in parallel).

[basssears]

Originally Posted by marty9876

Regarding the Victron controller, this has me pretty confused from the manual:

3.2. PV configuration
The controller will operate only if the PV voltage exceeds battery voltage (Vbat).
● PV voltage must exceed Vbat + 5V for the controller to start. Thereafter minimum PV voltage is Vbat + 1V.
● Maximum open circuit PV voltage: 75V.
The controller can be used with any PV configuration that satisfies the three above mentioned conditions.

The 'start' part where PV voltage must be +5v from a Vbat. Not sure what start means, in my particular setup I'll have 2x 100w panels @ 17.7v. Current thinking is to stay parallel - conservative approach plus I may add 2x 50w panels and mixing those in series would greatly reduce the output from my understanding (mixed watt panel same volts scale linearly in parallel).

My take on this:

Stay you're batteries are half gone (12.2v)... your panels would need to be putting out 17.2v or higher before the controller would start feeding amps to your batteries, and once the controller starts feeding amps to your batteries the panels would have to keep putting out at least 13.2 volts to have the controller continue to feed amps to the batteries.

Is 17.7v the max your panels will put out? Sounds like at that max they would have to be operating pretty good conditions before they would even "start" that controller.

This is why I am going to standardize around 24v not 12v system... open circuit voltage on my panels is 22.7 so as a "24v" system (i.e. running in series not parallel) open voltage is 45.4, meaning I don't have to have a hugely efficient setup going to get this controller started.

[john61ct]

The high-voltage MPPT controllers are designed to work with higher-voltage input from the panels.

If you are using lower-voltage panels, PWM is going to be more efficient in most real-world conditions.

Or put the panels in series, only downside is partial shading impact, but same with fewer large panels.

Thanks for bringing to my attention that only the bigger Victron models have temp sensing at the battery.

Note LiFePO4 banks don't use temp compensation at all, and overheating protection is not relevant with mobile solar's low amps.

[john61ct]

Originally Posted by marty9876

● PV voltage must exceed Vbat + 5V for the controller to start. Thereafter minimum PV voltage is Vbat + 1V.
...
Not sure what start means

Means if the charging voltage setpoint is 14.8V, the controller won't even try to start any charging until the input voltage from the panel(s) reaches 19.8

If clouds later come along, it won't stop attempting to charge unless the input voltage drops below 15.8.

[basssears]

Originally Posted by john61ct

Means if the charging voltage setpoint is 14.8V, the controller won't even try to start any charging until the input voltage from the panel(s) reaches 19.8

If clouds later come along, it won't stop attempting to charge unless the input voltage drops below 15.8.

OK so start is based on charging voltage setpoint, not current voltage of batteries... good to know, thanks for clarification.

For OP, sounds like you already have your panels, but you can always reconfigure, a mix of series and parallel could work well for you... for our needs instead of putting two 100w 12v panels in parallel we're going with two pairs of 50w panels in series which we then connect in parallel, so end result is two 100w 24v panels in parallel.

[john61ct]

Originally Posted by basssears

Not been able to clarify why one would pay the extra $40 for the Victron MPPT 100/15, the difference between accepting 75V and 100V open doesn't seem to make much difference to me at least, don't ever envision going above 24v but even so 48v is still well within the 75/15.

Do you think the voltage referred to by the first number is for the system, battery output? It's not.

Adding up the maximum Voc "shorting voltage" of the panels in series, that number is the hard limit of the controller, exceeding which may damage it.

So the 75 series can handle no more than four of your 17V panels in series, while the 100 can go to five maybe six.

The second number amps is OK to exceed based on the (mostly theoretical) Ioc panel rating, that is just the maximum OUTPUT you'll get, so sacrificing some (very occasional) PEAK amps by putting in more panels, will result in a much higher AVERAGE power output.