Voltage between battery and charge controller with load

[Atlas123]

Hello,

I am looking to set-up a solar set-up at home just for 12V LED lighting (100W panel, no more for the time being). Still learning and testing.

I have been doing some testing to check the charge/discharge process and battery capacity, with the main goal of making sure that the charge controller does not burn out my batteries (at present second hand 12V lead acid batteries 60amps). I am worried the charge controller is not doing its job but also the voltages I get are strange and I must be understanding something wrong (I did a similar set-up with a 7amp battery and 0.6amp load, 10amp charge controller and it seemed to work where the CC would turn off the load).

The set-up is:

solar panels not connected--> Battery (lead acid 12V 60amps)--> 10amp fuse --> volt/amp meter (with total amps recording, 100 amp capacity) -->Charge controller (20amps capacity, 11.1 low Voltage disconnect) --> Load: car light 12V (4.5 Amp pull).

The resulting voltage after measuring the battery under load at the end of the discarge is:

Battery has 11.6V (mesured directly to the battery)
Volt/amp meter shows10.9V
Measuring the battery poles on the Charge controller I get 10.9 (same voltmeter as the measurment directly on the battery)
The load pulls aroud 4.2 amps
--> Result: The charge controller is not "turning off" the load to protect the battery from full discharge. I have not gone further to not damage the battery.

Questions:
-Why is the voltage of the battery different than the battery voltage in the charge controller (battery input)?
-Why is the charge controller not shutting down the load at 10.9 if the LVD is suposed to be 11.1?

Thank you for your help.

Marc

[Sunking]

Your first problem is your battery is dead. A fully charged battery should be 12.6 volts. 11 volts under load has gone beyond completely discharged. Unless your charge controller has a load output, and you connect the load to it, the LVD does not work. Very few ever use the load terminal on a controller because it is so low power and cannot supply much power.

Other than that nothing is wrong except poor design of using too small of a wire. So you have 3 things kicking you in the but.

1. Your battery is dead to begin with. You should never see it go below 12 volts unless abused.12.6 volts is charged, 12 volts is STOP. Your battery is beyond dead.

2. You most likely have very small wire and incurring voltage drop. First drop is between the battery to controller. Your math indicates 11.6 - 10.9 = .7 volts or 6%. Should be less than 1% or .1 volts. Second loss is in the Controller LVD circuit itself which you cannot control other than keeping current low. Third loss is in the wire between the Controller Output and your load device. Total loss should be kept to 2% total, you are around 10%

3. More of a guess but you have your battery and load connected to the wrong terminals. Battery goes to battery out on controller. Load to the load terminals if you use it but most do not, and just connect the load directly to the battery terminals.

Too late to worry about the battery when you let it go below 12 volts.

[inetdog]

You have not described the Charge Controller (CC) that you are using, but almost all of them are designed with either two or three sets of terminals. All standard CCs have the first two.

1. DC input (from solar panels ONLY). This allows the CC to control how much current is going from the panels to both the batteries and to loads connected directly to the batteries.
2. DC output to batteries. This is where the CC can determine what the battery voltage is and how much current it should allow through from the panels. Large loads, such as inverters, should be connected directly to the batteries, in the same place that the CC is connected.
3. (optional) Load output. This is a controlled DC output that can serve different purposes for different CCs and settings.
It often provides DC to one or more small loads under a selectable set of circumstances:

A. Whenever the battery voltage is above the Low Voltage Cutoff (LVC). This, in theory, protects the battery, but in fact almost always too low a voltage to serve this purpose.
B. Whenever the panels are charging the battery and the voltage is in the Absorb or Float stage and therefore the panels may have current to spare to go to a load.
C. On a time of day basis, using the panels as an indicator of when sunset has come and turning on the load, presumably LED lighting, for a programmable length of time, as long as the battery voltage does not drop too low.

A CC with option A enabled should shut down the Load terminals not when the load output is low but when the DC input voltage (battery) is low.
It is not clear what options, if any, you have programmed your CC for, but it is probably not installed correctly.

If you want to use the (worthless) LVC function of the Load output to control larger loads, like and inverter, you need to use the Load output to control a relay which switches the actual large load on and off.

Most CCs will not function properly until they have first had their DC output connected to the battery and then their DC input connected to working panels.

This is the most likely single cause of any symptoms you are seeing.

PS: What Sunking said.....

[Atlas123]

Hello Sunking, Inetdog,

Thank you both for your comments.

Inetdog, your point on: “Most CCs will not function properly until they have first had their DC output connected to the battery and then their DC input connected to working panels.” Is very interesting as I had not connected the CC to solar panels yet, as only using it at present to manage the over discharge voltage (will explain further down). I will try this. I have another CC that works properly with the LCD at 11V that was connected to a solar panel previously (but I could not use here as I think the load was too much for this new test).

I will try to add some more information:

-Details on the charge controller are:
-PWM CC with 3 sets of terminals (Solar panels, Battery, load)
-Work Voltage 12V (and 24V)
-Rated charge current: 20A -Rated load current: 20A
-Over Voltage protections: 14.8V -Over discharge voltage: 11.1V

-Cabling across the set-up: 2.5mm (I believe this is 10AWG?) Shouldn´t this be enough?

-The batteries a recovered car batteries (60 amps) that cost 5$ the first time (once they “burn out” I can just swap them at the garage for ones that can keep a charge at 12V). I expect their life to be of maybe 1 year+ (maybe more) with daily use of 20amps with LED lights (and an input of 20amps daily if there is sun). Investing on deep cycle batteries is not an option.

-The reason I am discharging the battery now (and wanting the CC to manage the low voltage) is to have an idea of its useful capacity (it is 60amps but will probably give only 30-40 amps) and to check if once the batteries are in parallel (planning to use 3-4 f them) if they will have a similar capacity. To do this the best option I can find is to use the LOAD teminals of the CC. and having the LVD making sure it turns off soon enough.

-This set-up will be parallel to the mains (not connected to mains) At present solar energy laws in Spain tax solar set-ups connected to the mains (LAAAAA MACARENA ).

-Expected normal load: 3-4 amps for 4 to 6 hours, Maximum load expected: 7amps.

-Managing lack of sun: Use of main lights if not enough battery and maintaining batteries with a battery charger if needed.


-How then to manage automatically a solar system where we do not want it to go below 12V? (please don’t say visually with a voltmeter… ) I thought the point of having a CC was not only to manage the solar input but the battery discharge? (and they tend to have a 11V LVD). Wouldn’t that destroy all batteries? Or are CCs designed only for deep cycle?

Thank you again!

[littleharbor]

All but the cheapest charge controllers should have an adjustable LVD. This experiment you are trying is destined to fail as you are using a CHARGE controller to do DISCHARGE control. Why you don't have a solar module connected is beyond me but if you want this setup to work properly get a known healthy deep cycle battery, make sure it is in a good state of charge before you connect it. connect the battery first. SECOND, connect your solar panel, THIRD, connect your load. Now if you have any adjustable load control set it no lower than 12 volts.
These 12/24 volt controllers sense the nominal battery voltage and decide weather you have a 12 or 24 volt bank then charge accordingly. If you start with a over discharged battery AND no charging source you are asking for problems.

[Atlas123]

All but the cheapest charge controllers should have an adjustable LVD. This experiment you are trying is destined to fail as you are using a CHARGE controller to do DISCHARGE control. Why you don't have a solar module connected is beyond me but if you want this setup to work properly get a known healthy deep cycle battery, make sure it is in a good state of charge before you connect it. connect the battery first. SECOND, connect your solar panel, THIRD, connect your load. Now if you have any adjustable load control set it no lower than 12 volts.
These 12/24 volt controllers sense the nominal battery voltage and decide weather you have a 12 or 24 volt bank then charge accordingly. If you start with a over discharged battery AND no charging source you are asking for problems.

Fair enough Re. the adjustable LVD, there is certainly something wrong in the set-up if the CC is not turning off the load below 11V.

The initial experiment with a smaller set-up succeeded and works well (load off at 11V). This worked well with a panel for its main function but also for the discharge part without a panel. I will guess you did not have the chance to read my last post and that you are not trying to be snappy with the "beyond me", as I explained I am trying to measure the capacity of the battery (with the tools I have), connecting the solar panel and having a solar charge would defeat that purpose and this setup has worked on a smaller set-up.

First time I see that the conection order would impact the working shape of the system, thanks for the info I will check that.

If I had done this with a deep cycle battery and killed it, we would all be having a laugh... My learning curve may be steep, but no need to waste money burning expensive equipment .

I started with a charged battery at 12.8V