Absorb Voltage

equis

Hello

My inverter is tripping at 14.1 volts
Could I set absorb voltage to 13.9 instead of 14.1 but extent the absorb time to 4 hours instead of 2 (or when only .5% current is going in)
Currently AGM's but thinking of changing to T105's


Thanks

Rhys

BB.

Re: Absorb Voltage

Welcome to the forum!

Regarding your inverter--Charging for an automotive battery is around 13.8 to 14.2 volts at moderate temperatures. Having an inverter that cuts out at 14.1 volts is a problem. And charging your battery bank at 13.9 volts or less will take forever (and is not really healthy for a deep cycle battery).

Most inverters will cut out at ~15.0 volts -- Which is still a bit on low end for voltage. There are some inverters that will operate in the 15.5 to 16.0+ volt range (15.0 to 16.0 volts is the equalization range for flooded cell deep cycle batteries--even higher in sub freezing weather).

Are you sure about the calibration of your volt meter (and batteries are good)--It has happened before that a DMM was flaky and reporting bad numbers (check the battery voltage on a car or two--Should be 13.8-14.2 volts--closer to 14.1 to 14.2 volts ideally).

In the end, either the inverter is not working correctly, or you should look for a replacement.

-Bill

equis

Re: Absorb Voltage

Hello Bill

Thanks heaps for the reply,
In my question I oversimplified it thinking it would help, but I have done the opposite, sorry about that. :-)

My inverter is actually 48 volt and the manual says it will take 58.5 volts
My absob voltage is set to 56.4 but sometimes during absorb my inverter will say DC_BUS_FAIL, apparently this is over voltage.
I thought perhaps just lower the absorb voltage but extend the time woudl be fine.
Not sure if lower voltage will damage the batteries.
My charge controller counts A/H so I can tell when it is replenished even at the lower charge voltage.

Thanks again

Rhys

BB.

Re: Absorb Voltage

Rhys,

Well--Mulitply all my voltages by 4x and we will be talking about your system.

What brand/model of inverter do you have? What brand/model/type of batteries?

58.5 volts is still pretty low.

No chance that you have bad electrical connections somewhere (i.e., really a low bus voltage due to poor electrical connection back to the battery bank)?

-Bill

equis

Re: Absorb Voltage

Hello

I have http://www.mppsolar.com/Photovoltaics/PURE%20SINE%20INV_CHARGER_LF.pdf the 6000watt unit
I have checked all my connections, same thing happened when I have a 12v system.

I think it may just surge a bit during charge.
I have cheap ebay AGM's 200a/h at 48v

Perhaps using T105's will fix the issue, I was thinking they may have more resistance than the AGM's (could be wrong)
The other option would be a larger bank, harder for it to surge above 58.5 volts.

Thanks

Rhys

BB.

Re: Absorb Voltage

Rhys,

You are getting deep into the details behind the systems... And I will give my impressions of what could be happening--Please realize that I am not an expert in anyway here and I do not know anything about your inverter system.

First, AGMs... They typically are charged at a lower voltage than Flooded Cell batteries (around 14.2/56.8 to 14.4/57.6 volts maximum) to prevent too much gassing (can cause catalysts to over heat, batteries to vent--And AGMs are not supposed to be refilled with water).

And, AGMs can have very high surge current support--Flooded cell are good for around C/2.5 (C=20 Hour capacity rate) and some AGMs are rated for as high as C*4 rate of discharge (or charge) -- Yes--15 minutes to dead (typical UPS specification for high rate discharge until computers are shut down or backup generator taking up load). Obviously, you need very heavy copper cables/busbars to carry such large currents--And for off grid homes, We tend to suggest C/8 to C/20 discharge rates as you want to have battery power for a few days--not just a few minutes of loads.

But then there is what works well for "typical off grid systems" (we try to be conservative here--So you may be able to push the envelope, we will try to give rules of thumb that will work as a starting point).

With a "typical" battery bank, we have been suggesting 100 AH (@48 volt battery bank) per 1kW of rated inverter output (and your loading). This also supports ~2x inverter rated output for surge loads. This number is, more or less a (conservative) requirement for flooded cell deep cycle battery banks.

With AGMs, they can, obviously, manage much higher surge currents so you could support a much lower AH capacity--But, there is the flip side for charging... While AGMs can also absorb very high currents during charging--It appears (my reading of what others have posted here) that the ability of AGM's to "sink" high current (surge during charging) is quite a bit reduced near 100% SOC--So much so that for some (few, many, all???) MPPT type solar charge controllers, that I would suggest that you have a similar 100 AH (@48 volts) per 1kW of solar panels too...

As near as I can tell, if you have a 6kW array and a 200 AH @ 48 volt battery bank, that when the MPPT charge controller does a "sweep" (basically trying different current levels from the array and measuring array voltage--Trying to find Vmp*Imp=Pmp for that time/temperature of array). It appears that such a "sweep" can actually take a battery bank >72 volts in some cases (faulted the inverter/solar charger system in one case). Using the 100 AH @ 48 volts per 1kW of solar array gives a large enough battery bank (even AGM) that it can absorb those surge currents (for short periods of time) without over voltaging the battery bank.

So, it is possible to have "too small" of AGM battery bank and "lose control" of the battery voltage in some charging conditions. Is that what is happening to your system--I have not a clue.

It is also possible that different AGM brands will react differently to charging current at 100% state of charge (Concord said theirs will increase voltage and not over heat the catalysts, outgas, and such).

equis wrote: »

I have http://www.mppsolar.com/Photovoltaics/PURE SINE INV_CHARGER_LF.pdf the 6000watt unit
I have checked all my connections, same thing happened when I have a 12v system.

What do you mean by 12 volt system? Obviously different hardware/system.

I think it may just surge a bit during charge.
I have cheap ebay AGM's 200a/h at 48v

Is this during Grid AC (or Generator) charging? Or this when solar charging?

A quick look at the MPPSolar system/inverter seems to be biased more towards AGM/other sealed batteries. Not high enough charging voltage for flooded cell (no equalization charging?).0

Perhaps using T105's will fix the issue, I was thinking they may have more resistance than the AGM's (could be wrong)
The other option would be a larger bank, harder for it to surge above 58.5 volts.

More AH of either type of battery may help--However, I would be very worried if the system (at least as documented in the glossy) is capable of properly maintaining a Flooded Cell Battery bank.

What is your configuration of this system (AC grid/mains? AC Generator? Solar Array? Whose Solar Charge controller--if any?). What are your loads like (do you need/use 6kW of continuous power--and possibly even more surge--well pump, shop, etc.)?

And, how are you using the system? Pure off grid? Whole home/shop UPS system for when grid fails (and are you looking for hours or days/weeks of emergency backup)?

Sorry, more questions than answers at this point.

-Bill

inetdog

Re: Absorb Voltage

BB. wrote: »

As near as I can tell, if you have a 6kW array and a 200 AH @ 48 volt battery bank, that when the MPPT charge controller does a "sweep" (basically trying different current levels from the array and measuring array voltage--Trying to find Vmp*Imp=Pmp for that time/temperature of array). It appears that such a "sweep" can actually take a battery bank >72 volts in some cases (faulted the inverter/solar charger system in one case). Using the 100 AH @ 48 volts per 1kW of solar array gives a large enough battery bank (even AGM) that it can absorb those surge currents (for short periods of time) without over voltaging the battery bank.

I suppose that if an MPPT CC did not have sufficient internal energy storage (between the input and output stages) to allow it to do a high current draw from the panels without over juicing the batteries it could have the side effect of raising the voltage to the batteries. But that would be a very poor MPPT design and not a controller I would want to buy.

Cariboocoot

Re: Absorb Voltage

My opinion, which probably won't be too popular, is that a 48 Volt inverter that shuts down from over Voltage before 62 Volts isn't very good.

Since it will take 58.5 Volts max, that's equivalent to 14.6 Volts "per 12 Volts" as it were. Most AGM's charge at 14.2 to 14.4 Volts "per 12 Volts" so the OP's proposed suggestion of switching to AGM's from flooded cells might just do the trick.

An expensive solution to be sure, but the other solutions would be equally expensive: changing the inverter, or somehow dropping the Voltage to the inverter from the batteries (as this appears to be an all-in-one unit that's not even a very practical idea).

Perhaps you should contact the company that made the inverter and see if they have any recommendations. Considering that a Trojan bank on it would want 59.2 Volts Absorb and a Surrette or Interstate bank 60 Volts, this doesn't seem to be a well-thought-out design.

Out of curiosity, is there a battery temperature sensor included in that unit? They can alter Voltage +/- 2 Volts or so under the right circumstances.