AGM batteries and MPPT controllers compatible?

[doive1231]

Hi,

I'm looking to create a small solar panel system (80w panel) and am looking at a 10A MPPT controller off ebay which has good reviews. However, it does not specify compatible battery types and I was wondering if an AGM battery of around 35ah would work with it? I searched around just before I was going to buy the controller and read that MPPT controllers will not work with AGM batteries unless stated. I have emailed the seller and am awaiting their response.

I am looking at an AGM battery because I don't have a place outdoors for batteries and it would be in my kitchen with some ventilation from a window. The panel will be running a 30w DC fridge for approx 6 hours per day in Spring/Autumn to 10 hours per day in Summer. I am in Manchester, south facing with a solar insolation of between 2.5 - 4.5 for the times of year to run the system.

The battery and controller are the last components to get but I haven't read many posts about AGM batteries and MPPT controllers. Thanks for any responses, Dave

 

[solarfred]

Your main problem is that the fridge is running 15 amp hours per day - based on 6 hours per day "on time" as it is running off a thermostat. You say 10 hours in summer which makes sense as I was working on something similar today and concluded 6 hours a day was about right.
Your 80 watt panel will output say 5 amps on a good sunny day. To just replace the 15 amp hours it needs to be at full chat for 3 hours. On a dull day you will be getting maybe 0.5 amps or less from the panel so it needs to work for 30 hours to replace the 15 amp hours. Now I have heard Manchester gets a lot of bad weather so your fridge will stop after about 2 days. When the sun is out you need to store enough capacity for the days when it is raining and you only have a 1 day reserve of power in the battery (if it is a decent deep cycle). Therein lies the problem. The system is undersized for what you want. There are also losses in any battery system which further depletes your reserve. Your MPPT controller will not make up the difference required as I am guessing the panel is low voltage so outputs 18vmp? A bog standard controller will do the same job. You need more panel and more battery. If you need a decent AGM deep cycle 80ah or 120ah for a reasonable price then I can help you out! If you buy Leisure batteries you are wasting your money.
Using the wrong controller with the wrong charging settings can cause problems with sealed batteries hence you need to ensure the controller is intelligent and can be set for the particular battery type.

 

[doive1231]

Thanks for your feedback. I'm beginning to realise that I will need a bigger battery for a few cloudy days in a row.

I've just had a reply from the seller and they have confirmed that the controller will work with an AGM battery. The decision now is based on price as this is a first foray into RE and I have a budget. The 35ah AGM battery is approx £55 and a 75ah sealed deep cycle lead acid battery (also from ebay) is also approx £55.

Therefore, assuming I can site the SLA battery in my kitchen too, it would seem I should up my capacity. But you say buying a Leisure battery would be wasting my money. How come?

 

[solarfred]

Leisure batteries are designed for starting engines and running electrical appliances for long periods. They are a compromise for say a boat or campervan where one battery can do both jobs. If you are on a boat you have low load electrical draws for extended periods but still need the capability to start the engine. The engine then recharges the battery when it is running. Same in a campervan.
You don't need to start an engine so you don't need a high cranking power battery but a deep cycle storage battery that can be drained over say 10 -20 hours to 90% of its capacity and the recharge it back to 100%. If you drain a leisure battery down to 90% it will be dead. Supplier would normally advise 50-60 % discharge. So your actual capacity is not 35ah at all. If you look up the specification chart it will show you the capacity over 10 or 20 hours normally. Deep cycle are made with thicker plates to store more power for slower release. They are designed for the job so invest in the proper kit.
People always buy leisure batteries because they are cheap. Check the weight and a deep cycle will weigh much more because it has more lead in it. Lead costs more. Decent deep cycle will be able to provide many more full cycles than leisure "equivalent".
Flooded lead acid are best for solar deep cycle work as they can be topped up and monitored. AGM and GEL are non spillable so popular for golf carts, buggies, boats etc. but need careful charging regime so as not to overcook them.
You really need to compare like for like and then you will see where the benefits lie. So check your battery 35 ah capacity - is it 10 hours for 35 ah or 20 hours for 35 ah. Then check another make for the same hours period? It may be very different.

 

[doive1231]

Thanks again. My experience is that I wish these battery types could be clearer. I've seen batteries listed as both leisure and deep cycle and there appears to be a gap in the market for sellers and manufacturers to specifically state suitability for solar projects. Leisure obviously covers more than one application and this is where solar newbies should be aware.


From your reply, (remembering that I can't keep a flooded top up battery in my house and my budget), I should go for a deep cycle sealed lead acid battery with more amp hours than 35 and rated at 20 hours. I have seen these from Lucas and Numax and will try to read up more about them. I hope to have everything in place by the end of next week and really this is an experiment to see the performance of the system and if it is possible so I will not be too disappointed should I not get perfect results. If my cheese goes off a day or too sooner, then I won't be too worried.


The controller I am looking at displays readings for power, voltage and current and so I will post back the results under different weather conditions soon.

 

[doive1231]

Up and running.

 

[Vegelen]

If you can, It would be very useful to other forum members if you could tell us some more information about the type of system you installed to run which appliance.
Which batteries did you choose? How much did they cost? etc.
What currents does your appliance typically consume in operation, how does this affect your battery charge in winter, spring and summer?
If you could return to the forums at a later date to tell others how your system is performing, what you've learned etc your experience would be useful to others planning on doing similar things.
These forums work best when knowledge/experience is shared.

 

[doive1231]

Hello,

I have figures for 16/3/2013 which was a bad day weather wise for the 80W panel. There were 4 weather conditions: 1) cloudy 2) sunny but panel in shade 3) sun visible through cloud 4) sunny but panel partly in shade. On the 17/3/2013 I have had sunshine at 2pm albeit weak. Figures are:

MPPT controller reading
Cloudy 0W
Sunny but panel in shade 5W
Sun visible through cloud 3 - 6.8W
Sunny but panel partly in shade 16 - 18W
Sunny 41 -45W

I chose a 110Ah sealed lead acid battery deep cycle (Numax CVX31-MF) £77 from Tayna (like on Facebook for £2 discount) and is kept (ventilated) indoors.
MPPT controller £29 from ebay
80W moncrystalline panel £70 from friendly green giant on ebay
Extra 4mm2 solar panel cable 10m £10

The panel is south facing inclined at 55 degrees. My biggest problem is shade in the morning which unfortunately is unavoidable but should improve up until mid-June. My appliance is a 30W DC fridge but the MPPT controller says it is drawing 35W in operation and I have only had it running less than an hour in total just at the moment. The battery is new and has stopped showing a wattage reading I assume because it is fully recharged. It is too early to draw any conclusions just yet as to running at the watt hours I require (I said 180wh at this time of year). 45W from the panel today is encouraging, as is power being created in shade and sun through clouds. It must be recognised that the panel will not produce power in cloudy conditions and this can be many hours. The days are over 12 hours long at the moment.

Total cost <£190. Ancillary costs (such as extra cable, cable grips, and so on) should not be forgotten when budgeting.

- Buy extra cable with MC4 connectors already attached or you will need to buy a crimping tool.
- Order your battery last to reduce chance of discharging while waiting for other components to arrive
- Cover your panels when connecting whilst sunny so that they don't produce power
- Watch videos such as Jonny's at AKT solar to help with wiring : Videos.

 

[solarfred]

Did you check the short circuit voltage and current of the panel when you had sun? You should get maximum VOC in any sort of sun. Then what are the amps readings on the charging circuit?
£30 is not a lot of money for a proper MPPT controller. You may have a PWM in disguise but hard to know from the outside.
The battery should be starting to struggle if we get no sun tomorrow (depending on how many times you open the door) so let us know what the readings are. The battery is a Calcium Lead leisure battery so may not fully recover if it is taken too low. I am not sure if that controller will disconnect or at what voltage it will do this but maybe you have information sheet. Please keep us informed.

 

[solarfred]

Sun!!!! As we had some sun this morning I decided to test my new batch of panels and get some images for you.
First off I took a photo of the sky. It was hazy sun. The level of light was variable because of this but improved a little as noon approached.
I then took a light level reading off the panel in W/M2 = 1130 (Kent) Angle of panel 50 degrees. Temp approx 8c.
The panel is an 80wp with label as shown
The serial number shows the flash test report: VOC=22.03 ISC= 5.12 VMAX= 17.48 IMAX= 4.83
The solar controller is a basic Chinese model and shows 5 amps charging current as per clamp meter.First reading is off the panel input to controller and then charging current into battery.







So the panel is preforming well in March. The lower temperature from STC gives good power output ie. pretty much maximum.
The equipment is not laboratory quality or recently calibrated for the record.

 

[doive1231]

Hi solarfred,

Your pictures look good. Re: my controller. You can look on ebay for 10A MPPT at the price I mention and see what you think. I have no way of knowing if it is a PWM at the moment other than I checked the feedback for other sales of this item and there are no flags to say otherwise. I will record the voltage and amps in good sunshine and post them. The instructions it came with show a DC/DC circuit in the unit.

The battery was recommended for solar by Tayna and has a 3-year warranty and I expect it to last longer and be adequate for my needs. The cheapest 110-120ah AGM battery was twice the cost and therefore I didn't think it would be cost effective for my small scale project. If you are selling batteries then you could provide a link and advertise your site/listings (if that's allowed) for others attempting similar or larger installations.

 

[solarfred]

Battery warranties are pretty worthless! How are they going to check you have not deep discharged too far and then not recharged correctly? If a cell goes down just after you bought it then possible replacement but after a few months if the battery fails to recharge fully you will not get a refund unless I am very mistaken. Elecsol give a 7 year warranty but AFAIK no one has ever managed to claim against it. You have to send the battery back to their testing facility at some cost and then if they decide it's your fault you have just spent the money you could have spent on a new battery on shipping the dud one. More lead more power. More lead more weight. A decent 110ah battery should weigh around 30kg. For testing purposes your leisure battery is fine but I am keen to see how long it will support the fridge at this time of year. Keep us posted.

 

[doive1231]

It's been cold enough that I haven't needed cooling at the moment and I have had to wire the fridge to a 12V DC timer as it doesn't have a thermostat and so doesn't switch itself off.

This seemed daunting beforehand but using a wiring diagram and trips to Maplin for connectors and wires, I have got it working and am in the process of testing by running the fridge for 10 minutes at a time.

There was some sun today and the controller briefly displayed 90 watts which I was very excited about but I don't actually believe is being produced by the panel but is the algorithm of the controller "testing" the panel for the best power point. The display showed 80 watts, 70 watts all the way down to 25 watts and then back up to around 35 watts at 2pm on a cold day with the sun coming and going.

When we get back to normal temps for this time of year I will initially try running the fridge for four hours per day and see how I get on. This is less than I planned but it will be trial and error until I see if the battery becomes drained. If not, I can increase the hours on the timer. I'm disappointed my panel is in shade in the morning and this will cost me and I've read the power may drop off in the afternoon so there are many factors affecting performance but I remain hopeful of having cooling from the system in the summer.

 

[solarfred]

If you invest in a current clamp meter (DC) then you can check the AMPS coming off the panel and going into the battery from the charger. Voltage off the panel will be fairly stable right across the light levels but it is the amps that generate the power. These will rise and fall with light levels. As you don't know the quality or even status of the controller I would not rely on the LCD display (which is nicking some of your hard earned power). You said it shows current and volts so you can cross check these and the meter will be invaluable. I would also get a multimeter and measure battery voltage so it does not go too low.

 

[doive1231]

Thanks solarfred. I will look into that. I certainly at this stage wouldn't totally rely on the figures from the controller.

There was some good sun at 1pm today and I got a stready power reading of 68.8W and have posted a picture and of the panel. There is a little bit of tweaking to do regarding position still etc. The controller says the battery is charged and is at 14.0V with no load. The battery also has a condition indicator with green for good and red for discharged.

Some specs of the panel:

Open-circuit voltage (Voc)	Av 21.6V, sd 0.3
Voltage at maximum power (Vmp)	Av 17.5V, sd 0.2
Short-circuit current (Isc)	Av 5.02A, sd 0.01
Current at maximum power (Imp)	Av 4.57A @ 12v
Maximum power at STC (Wp)	80W ±5%
Operating temperature	-40oC to +85oC
Maximum system voltage	1000Vdc
Power tolerance	±5%

Controller self use current <8mA. Running the fridge for 15 minutes the load is 32.4W and the battery is at 12.4V.