Generator starting voltage

[STEVEBROOKS]

This might be tough to answer (due to several variables) but maybe somebody has experience with it, so here goes:



Currently, the only way I have to monitor battery condition is via an inside-mounted panel that has individual "dots" that light up to tell me the batteries' supposed condition. I have no idea how much battery voltage each individual dot is correlated with. Instead of relying on those dots, I plan to install a digital volt meter beside the existing panel to show the batteries' actual voltage (two group 24 batteries). I'd like to know what anybody thinks is the minimum battery voltage I can go down to before I won't be able to start my generator (an Onan 4500).



Thanks,



Steve

 

[BK]

Steve, it's a tricky question, more so cause there's more to it than that.

The big thing is what is the voltage during cranking, and if that maybe the last crank attempt you get out of the battery.

The panel meters aren't quick enough to catch the low dips during cranking.

These panel meters are more so there to tell you the voltage that battery is sitting at during charge or when its just sitting there.

There are special devices out there that monitor the start batteries for commercial /critical applications. Their sole purpose is to capture the battery voltage dips during cranking, and provide a warning that that may have been your last or close to last crank out of the battery.
Technically 10. 2vdc is the low end vdc for the battery (during a discharge in some length of time). But cranking is different, some of the devices watch for 9. 6vdc during the cranking dips, and at or below that call it a problem.

Now of course, if the battery is sitting at 11vdc, cause it's not charged, yea, yea the panel meter will let you know, and yea you wont crank the genset.

But if the battery is on charge and floating at 13. 6vdc, but the battery is a bucket of water, your seeing the charger voltage = you still see 13. 6vdc and truly dont know the condition of the battery.

In trying to use a panel meter, 12. 6 volts is the normal open terminal voltage of a car battery (off charge), (2. 1vpc x 6 = 12. 6vdc for the 'monoblock' ), and or under a light load ( couple of amps to bring down the battery from a float to terminal voltage).

The only way to truly test a battery is under "system" load.
Those 100amp load testers are a nice way to go.
And for your size gen set is a nice load test.
Battery Load Tester - 100 Amp, 6v/12v

On testing I've done on a 5500w gen set (from fresh fully charged AGMs), I've seen the initial slug of current for about 50ms-100ms be about 300-400amp, and than rolling starter current right at 100amps or so.

Hope that helps...

 

[STEVEBROOKS]

BK,



Thanks for the info. Yea, there are definitely a lot of variables involved in this. I try to run the generator only when I really need to, and go off battery power as much as possible. I'm just trying to find some lower limit, with a little padding added in, that I can go down to and still start my generator. I don't want to have to rely on the trial-and-error method if I don't have to...



Steve

 

[mrtrombley]

Reading the battery voltage before trying to start the generator really won't tell you much. You need to know how much cranking power is still in the batteries. The gen takes a lot to turn it over. I found out, that even when plugged into shore power, the gen didn't have enough kick to start it if the batteries weren't good. My two batteries were old. Replaced them with one new battery and it started first try. Interesting that the battery needed to be strong even when plugged in!

 

[BK]

It's the initial hit of current required, the onboard DC converters just can't do it.

The initial current slug for pulling in the bendix and getting the starter motor turning is huge, about a 300-400amp hit on the 5500kw genset I was testing with last.



Than about 100amps rolling.



If you cant supply that initial hit, it just isn't going to happen.



Like starting a compressor on your AC with too thin of an extension cord to the trail, or too light duty of a hook up.



Finally found my report screen captures ... . here's a starter hit, it's 100amps per CM (vertical square) ... center line of the grid is Zero amps (inline with the "A").



#ad

 

[BK]

And as you point out (and mrtrombley), you can be running for x number of hours on those batteries, w/o a good way of quantifying how much engery you've pulled out of them, even before seeing the terminal voltage fall below lets say 12. 2vdc .

And depending on lots of other things, where / how far down you've discharged the battery can be different ... .

-length and size of wire run to the genset (each application will be effected by it's own wiring, short and larger the gauge wire, the better)

-over all health of the batteries (changes each charge and discharge cycle)

-temperature
(colder than roughly 72degF and hotter than roughly 72degF, and you the capacity of the battery is de-rated)

-health of the starter itself (there's a commercial unit coming out on the market that monitors the starter's health during monthly PMs, and will alert to a change in current starting characteristics and warn of a failing starter)

will greatly effect where the battery wont be able to supply that initial hit and rolling current.

Best bet is a lone starter battery for the genset itself. . or a pull cord ! LOL ...

 

[STEVEBROOKS]

Thanks again to all for the info.



Steve

 

[Wertles]

I ran into this issue with my trailer. I had one of my batteries fail at 4 months. I found that with the truck and a set of jumper cables, I could jump start the generator but it was a HUGE PIA. That prompted a trip to the local store and found a good Jumper box. I should have had this years ago since it is a breeze not to jump start anything, especially cars that are parked in the garage. I now find piece of mind knowing that no matter what, I can always start my generator.

 

[Kirwin]

Genreator starting voltage

This may or may not be helpful to your question. I have a AGM battery set up to start my 12 KW QD Onan generator mounted in the back of my truck. The battery cables run forward along the right side frame rail to a custom built battery box approximately where the fuel filler would be on the right side of the truck under the bed. For 3 years it was never a problem. Of course, on a cold day when my wife wanted to dry her hair, the generator would not crank long enough after preheating the glow plugs to start the generator. The control unit shuts down the start cycle at 9. 5 volts to protect the electronic componets in the generator control circuitry. Its not just the voltage of the battery itself, it's the voltage drop in all the connections/cables between the generator and the starter. Ultimately I moved my battery closer to the terminal block on the generator; not only does it start in cold weather but it cranks much faster in all conditions.

You can get a Xantrex Link 1000 battery monitor to measure amp-hours charged/discharged to get a better handle on how much energy remains in your battery bank. I use one and it is very accurate as to percentage charge remaining in the battery bank.

 

[Chrisw]

i have found the same thing as mrtrombley even with my converter making dc power if you don't have a strong battery the gen will be hard to start or no start.



with a little effort and 1 spare battery you can place your gen on its own battery and never have to wonder if its going to start. I have thought about doing this to my camper but so far, I have not taken the time to see what it would take to make this change.