Ah Usage and Generator Runtime Calculations

[Trickle Charge]

I suppose this thread could also be called "Batteries and Charging 101".

So if I've used 250 Ah and my generator outputs 25 amps, would it actually take 10 hours of runtime to make it up or is it the 25V and 110 amps that the inverter is indicating that counts, making it about 2.5 hours of runtime, give or take certain factors?

Cheers
tc

[Vic]

Hi T C,

The recharge and discharge of a battery is all relative to the AH going into and removed from the battery.  So,  if the charger that you are using can actually deliver 110 A for hours continuously,  then this is the value that is important.  Even if the charger cannot deliver that amount for hours,  it is still the charge current delivered into the battery that counts.

There is also a battery recharge efficiency -- about 85% for FLAa,  and over 90% for AGMs.   These factors do vary,  and depend on how the current was removed from the battery -- slowly over days,  of quickly in hours etc.  And,  there is also a bit of inefficency caused by self-discharge.

So if your charger can deliver 110 A for hours  this is the value that you would consider.  To fully recharge the battery,  four of so hours would normally be required,  as the charge current tapers as the voltage rises.  This is especially pronounced in the Absorption charge stage.   Trying to fully recharge a battery  only from a generator is not too efficient.  Often,  it can be a good strategy to Bulk on the generator,  and try to let PV or other RE power source try to finish the charge.  YMMV,   Vic

[Trickle Charge]

Hi Vic,

Thanks for the info.

The Magnum seems quite happy pumping 110A for at least two hours, as is the 6kW Ecogen running out 240V/30 plus amps to it.  I did get a nasty smell from the inverter while I ran a rather heavy load during a bulk charge from the generator, but it's a new installation and the smell was some of the transformer resin baking.  Other bulk charges since then haven't produced the odour to anywhere near that extent. Then again, I'm not going to run all my appliances while that's going on either...  lol

There is a PV array with nine 235W panels on one Classic charge controller and three 245W panels on another,  but on days where it's been particularly cloudy or downright raining I can produce barely more than 2kWh.  So I'm trying to get a feel for what I see as battery bank voltage drop and AH usage and then guaging that with a resonable run cycle for the generator so that I'm not running it for four hours, to be mindful of my neighbours during low PV output days, and to reduce losses at both ends.

The installers have entered a gen start voltage of 23V and a LBCO of 22V for an eight 12V AGM battery bank wired for 24V and a total of 980 AH.  After two hours bulk charging the bank with 27VDC/110A from the Ecogen (it may go into an Absorb Charge by that time but there's still 110A entering the bank), I get a net bank charge of about 25.5VDC after stopping the Ecogen.   I suppose I'm usually stopping it before a complete absorb charge where the current drops going in but after a usage of 250-30 0AH I'm back down pretty close to 23.0V.  I'll have to chart this to get a better picture but I'm starting with these ballpark figures.

So I'm inclined to conclude that 25.5V down to 23V is about 250-300 AH.

[vtmaps]

I think you are in trouble or headed for trouble.  When you draw large current from a battery its voltage sags, and then rebounds some after the current draw stops. 

When setting a LBCO or gen start voltage, you must consider the sag and the time at that sag.  For example, if my battery voltage is 25 and I turn on my table saw, within a minute the voltage sags down as low as 23 volts.  When I stop the saw the voltage goes back up to 25 (or nearly so) within a few more minutes.  The 23 volts does NOT reflect my batteries SOC.  Now suppose I leave a 50 watt load on my batteries with no charging source.  This time, when the voltage gets down to 23 volts it means something: my batteries have been severely over discharged.

What are the conditions under which you measure 23 volts?  It that voltage reflects your battery state of charge you are in trouble and need to get a long long absorb going.

Also, you have AGM batteries.  They have less voltage sag than my flooded batteries, meaning its more likely that your 23 volts is real.   One more thing... I think you should have your gen start and LBCO set at higher voltages.  My LBCO is 24 volts for 60 seconds.  Unless my battery is near full charge it means I can only use my tablesaw for a minute without shutting down the inverter.  That is the price I pay for protection from leaving a light on and draining my batteries below 50% SOC.

--vtMaps

[Trickle Charge]

Hi vtmaps,

Thanks for the reply.

I think I'm headed for trouble too then.

Basically when I see close to 23.0V I'm referring to a readout after a day or two's consumption, depending on appliance use, not for a dip in voltage under load.  I've seen that sag you referring to and I understand that, like the battery voltage of a car being higher than when the engine is running.  Here I'll run the generator for a time, less than two hours usually, and the readout for the bank is say 26V and at some number of hours later it's dropped down to below 24, closing in on 23V. 

I've been trying to wrap my head around this drop in voltage and AH usage but clearly I'm going to have to run the Ecogen into an absorb charge asap.  I'll also discuss the gen run and LBCO settings with the installer.  I have to keep my hands a certain distance away from things due to warranty. 

I have the ME- BMK unit installed.  That's where I get my AH usage values.

[vtmaps]

I just reread this thread and I see that you have 4 strings of AGM batteries in parallel.  That is a prescription for disaster.  It is difficult to get the current to divide equally between the 4 strings. The problem is especially acute with AGM batteries because of their low internal resistance. 

Diagnostically, you have two routes: 
1) get a DC current clamp meter and measure the current in each battery string while charging and while discharging.  You may even find some current flow when the batteries are 'resting'.  This would be because one string is discharging through another string.
2) disconnect three battery strings and try to charge just one string.  You can do that with each of the 4 strings and see if you have a string or two that is not behaving.

For the future you should try to set up a single string of larger capacity batteries.

--vtMaps

[Trickle Charge]

Thanks vtmaps

I've got a clamp on meter at home so that'll be interesting.  As for the bank of 8, I left that up to the designer...

I'll post more detailed info as I chart it over the next while.

TC