Inverter shows lower voltages than charge controller

SkinnyCow

Hi,

I have a Xantrex prowatt 2000 connecting to 175Ah battery using 4/0 cables (less than 2 feet). I use 250A fuse from battery to inverter. My charge controller also connects to the same battery terminals and it always shows 0.2-0.3 volts higher than what is shown on the inverter. For example, when the controller shows 12.68, the inverter show 12.4. Battery, cables, charger and inverter are all brand new. Is this the inverter problem or the battery?

Thanks in advance.

Cariboocoot

Re: Inverter shows lower voltages than charge controller

Welcome to the forum.

It is unlikely to even be a problem: the accuracy of Voltage readings for both the charge controller and the inverter is probably the cause. The difference you report is around 2%, and frankly the meters aren't that accurate.

Get a good DMM and measure the Voltage at the controller, the battery, and the inverter under the same conditions and see how much variation there is. Then remember this: the charge controller measure Voltage on its output terminals and this will be higher than battery when it is producing current then when not, the inverter measures Voltage at its input terminals and this will be lower than battery as the load demand increases.

With no input to the charge controller and no output from the inverter you stand the best chance of seeing all three points reading the same Voltage on the same meter. But keep in mind it can vary at the same point from moment to moment. Small variations are not a big issue.

For the controller it is important that it enters Absorb stage when the battery is really at Absorb Voltage, not a couple of tenths below it.

For the inverter it is important that the Low Voltage Disconnect does not activate under maximum load when the battery is really high enough to power it.

Both of these are functions of V-drop, which involves Voltage, current, wire size and length.

SkinnyCow

Re: Inverter shows lower voltages than charge controller

Thanks Cariboocoot for quick reply. I had no input to controller (no PV power) and no output from the inverter-nothing plugged in. The reason I'm sure the inverter is lower because I also have another Xantrex prowatt 600 connected the same way and it shows the same voltage as the controller does...controller shows 12.64, prowatt 600 shows 12.6.

Could you also recommend me a good DMM?

Cariboocoot

Re: Inverter shows lower voltages than charge controller

SkinnyCow wrote: »

Thanks Cariboocoot for quick reply. I had no input to controller (no PV power) and no output from the inverter-nothing plugged in. The reason I'm sure the inverter is lower because I also have another Xantrex prowatt 600 connected the same way and it shows the same voltage as the controller does...controller shows 12.64, prowatt 600 shows 12.6.

Any two of the same controller or inverter can give readings this different when connected to the same system.

Could you also recommend me a good DMM?

What can you afford, and what is available?
Fluke meters are top end, but you're looking at hundreds of dollars.
And sometimes you get lucky, like the cheap one I got out of Princess Auto for $10 that will read all kinds of things including 20 Amps DC (good for panel testing).

BB.

Re: Inverter shows lower voltages than charge controller

Is this an AGM or other non-flooded cell lead acid storage battery?...

Your 2kWatt inverter will basically flatten a 175 AH battery bank in an hour (or less). Pretty stress full and that high of discharge rate is usually only used on UPS (uninterruptable power supplies) for battery backup until the computers are shut down or the backup generator is started.

-Bill

Cariboocoot

Re: Inverter shows lower voltages than charge controller

Bill;

He says there's no output load so it can't be from taxing the batteries.
Although 175 Amp hours is really too low for a 2kW inverter run 'at strength'. Anything over 500 Watts will cause significant problems.

SkinnyCow

Re: Inverter shows lower voltages than charge controller

BB. wrote: »

Is this an AGM or other non-flooded cell lead acid storage battery?...

Your 2kWatt inverter will basically flatten a 175 AH battery bank in an hour (or less). Pretty stress full and that high of discharge rate is usually only used on UPS (uninterruptable power supplies) for battery backup until the computers are shut down or the backup generator is started.

-Bill

It's AGM deep cycle. My frig ran overnight and the battery was down to only 12.6 volts(reading on the controller) and 12.3 volts (reading on the inverter) - still well over 70%. This is the only thing I plan to use for so the 175Ah should be fine. My small frig consumes less than 350kw/year-average out about < 1kw/day which is consistent with what I observed so far.

I feel relieved reading all the answers now! So my set up will works for my application. I'll keep them plugged in and will report back if there's problem. Thanks every one.

BB.

Re: Inverter shows lower voltages than charge controller

Remember, if the inverter is on 24x7, it too will draw ~10-20 watts of power just being "turned on". You will have to add that to the refrigerator load (batteries and solar array).

As you head into winter--You will have to keep an eye on the system... During very dark/cloudy/stormy weather, you may see 5% of rated power from the solar array (i.e. almost nothing). Using a generator or grid power would be needed to keep the AGM's from being over discharged and killed.

-Bill

gww1

Re: Inverter shows lower voltages than charge controller

I have three cheap muti meters all read a volt or two different. All from different manufacturers (harbor f, sears, wally world). Both cc's read different till voltage compensation adjustments were made. The cc's are still .2 volts higher then the inverters but I got the cc's together so they would work together. I just remember they are .2 higher then the inverter. You should be able to compensate in the cc's if you decide which one is right. I just watch the temp of battery and sg's to decide if I am close on my voltage settings and then just don't worry about it.

Good luck
gww

Les Nagy

Re: Inverter shows lower voltages than charge controller

SkinnyCow wrote: »

Could you also recommend me a good DMM?

I started a thread for this subject:
http://forum.solar-electric.com/showthread.php?24789-Which-mulitmeter-My-opinion-for-those-looking-for-something-reliable

DOB

My inverter suddenly started seeing input voltage less than the accurate voltage, for instance my input on stabiliser is 200-220volts but it appears on the inverter as 135-140volts why ?