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Author Topic:   Inverters and batteries
kingfish posted 08-26-2005 08:57 AM ET (US)   Profile for kingfish   Send Email to kingfish  
I have recently purchased a 700 watt (continuous) inverter for my Outrage 22 so I can make coffee in the morning when I'm overnighting on board. My battery configuration on board is a fairly powerful AGM (deep cycle) battery and a fairly powerful lead/acid cranking battery, both group 24 size, on a battery switch. Reading the literature, it sounds like inverters like deep cycle batteries better than cranking batteries, so when I've used the inverter on board, I've atached it to the AGM battery, although I've usually got the battery switch turned to "both" anyway, arranging the two batteries in parallel. Works just fine, and the coffe is great.

Here's the question: At home I have a couple of big (100 amp/hour +/-) group 27 gel batteries and a couple of standard but higher end group 24 lead/acid cranking batteries. I keep them all charged, and they are charged now, all reading between 12.6 volts and 12.8 volts. But when I connect the inverter to either of the gel batteries, without anything plugged into it drawing power, the low voltage alarm goes off and the low voltage LED lights up. Power is not available through the inverter under those conditions. When I hook the inverter up to either of the cranking batteries, the under voltage alarm does not go off, and the inverter works just fine, even with a coffee maker plugged into it and running.

So what am I missing here? I've tested these batteries and hook ups multiple times and always get the same results. The inverter is supposed to give an alarm and shut down if below about 10 volts and I think above 15 volts. But all batteries read between 12.5 and 13 volts and I can't figure what gives. Is there some other characteristic besides voltage that an inverter senses (like amps?), that when not in the desired range causes the low voltage alarm to come on?

Any insight would be appreciated.


high sierra posted 08-26-2005 01:10 PM ET (US)     Profile for high sierra  Send Email to high sierra     
Kingfish, when you are connected to the starting batteries, they are designed to provide a large amp draw and will draw down fast. On the Gel batteries and most deep cycles they are designed to release much less amps over a much longer period of time. Sort of like a restriction on the gels to make them provide power longer. I believe your systems detect the restriction on the AGM's and is telling , there isn't enough amps. Longe range guess. High Sierra
jimh posted 08-26-2005 01:44 PM ET (US)     Profile for jimh  Send Email to jimh     
As far as I can imagine, the inverter device can not sense what type of battery it is connected to other than by the voltage. Well, perhaps if it were really sophisticated it might put a load on the source voltage and see how much the voltage drops. If it thinks the source is not stiff enough to work, it might sound an alarm. But on a $700 inverter, I would be surprised if it were that sophisticated.
kingfish posted 08-26-2005 06:16 PM ET (US)     Profile for kingfish  Send Email to kingfish     
Thanks high sierra and jimh-

It's beyond me (which doesn't take much re: electricity), but something is going on with the gel batteries I don't understand. I have forwarded my observations and question on to the inverter manufacturer. I don't think it is a very sophisticatd inverter either at less than $100.00, and I don't think it's *only* the difference in the make up of a gel battery as opposed to a lead/acid battery either. Partially maybe, but I'm wondering if my gel batteies have lost some of their capacity that I can't measure with just a voltmeter, but somehow the inverter can. I think the no-load draw of the inverter is something like 0.5 amps, and it's hard to believe the gel batteries aren't kicking that much out...


Swellmonster posted 08-27-2005 01:15 AM ET (US)     Profile for Swellmonster  Send Email to Swellmonster     

*** What are you doing running on both batteries? I know their are a few articles on this... One battery can overcharge, while the other is catching up which is another issue...
Last year I bought a 400 watt inverter to plug in some rope lites to put on my boat for a christmas parade. I plugged the invertor into my vehicles plug lighter and plugged 3 strands of lights in without the motor running, no problem, the invertor worked great. So I rigged up 3 strands of blue lights around the console and up the VHF antennae and I was getting ready to show off and chill with the blue lights and the invertor just would not take the load. I feel the pain for ya :)
Alternators usually charge 13.5 - 14.7ish at best, a little over 1.5v per liquid cell at battery level.

Can you brew coffee in your smooth riding SUV?

Swellmonster posted 08-27-2005 01:16 AM ET (US)     Profile for Swellmonster  Send Email to Swellmonster     
By the way, was that a "I have recently purchased a 700 watt (continuous wave) inverter? :) LOL!!
kingfish posted 08-27-2005 09:20 AM ET (US)     Profile for kingfish  Send Email to kingfish     
Hey Pat-

Continuous power output (as opposed to peak or surge power); but continuous wave is ok too I guess...I did find that these are modified sine wave inverters typically, but if you want a pure sign wave inverter so your computer and all of its accessories couldn't tell that it's not hooked up to the grid, you don't get 700 watts for $100.00 any more.

The whole thing about two motors and two batteries and how to wire and switch and protect the batteries under various conditions is currently over the horizon of my electrically challenged world...someday I may get a handle on it, but by that time I'll probably have forgotten why I was trying to do it to begin with.


kingfish posted 08-27-2005 09:27 AM ET (US)     Profile for kingfish  Send Email to kingfish     
Sine, sign, what's the difference, eh?
Jerry Townsend posted 08-27-2005 01:51 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
John - there is a big difference - the word sine refers to an alternating voltage, in this case, where the voltage increases smoothly to a positive maximum and the decreases to some negative maximum and then to the postive maximum,

Some alternating systems use a 'square wave', where the changes between negative and postive are abrupt changes. This square wave system is cheaper to make and will work well for lights - but is hard, at best, on virtually everything else - such as electronics. When you are thinking about inverters - be sure and note the output wave form - either a sine wave or square wave.

And remember - plugging an inverter into a cigarette lighter - the inverter takes power even without a load - so an inverter plus lights could trip the cigaretty lighter breaker. ---- Jerry/Idaho

kingfish posted 08-27-2005 02:57 PM ET (US)     Profile for kingfish  Send Email to kingfish     
Hi Jerry-

Thanks for the response. I was attempting some apparently ineffective light humor about my mispelling of "sine" in one of the two instances in my previous post...

The reading I've done so far indicates that "true sine wave" or "pure sign wave" inverters are the ones that you want for most situations where electronics of any sort will be operated. One of the articles said that the term, "modified sine wave inverter" is a misnomer invented by some marketing guys to describe what really is, as you have pointed out, a "square" sign wave, or really not a sine wave at all. I think for my intended purpose, to operate a 500 or 600 watt 4-cup coffee maker, I'm probably OK, don't you think? My 700 watt inverter hard wires directly to battery terminals with some pretty heavy gauge wire; I'd guess at close to 4 gauge, certainly no smaller than 6 gauge.

Here's an interesting fact I just discovered, that probably sheds some light on what's going on here. Both the group 27 gel battery and the group 24 cranking battery read close to 12.8 volts without the inverter attached and turned on. When I attach the inverter to the gel battery and turn it on, the battery voltage reading drops down to 7 volts +/- while the inverter is drawing power from it; it goes right back up to close to 12.8 volts as soon as I turn the inverter off. When I attach the inverter to the cranking battery and turn the inverter on, the battery voltage only drops about 0.05 volts, to about 12.75 volts while the inverter is drawing power from it, then bounces right back up to 12.8 volts when I turn the inverter off.

So is that a sign that while the gel battery can be charged up to 12.8 volts, apparently a full charge, it no longer has any poop behind it (poop being amps, I think), as if it has become damaged? Is there some kind of load test that can be performed to find out if this battery is just junk now, or what? It was an expensive battery - I think it was actually a 120 amp/hour battery, and I'd like to find out what's up with it before I arbitrarily pitch it.



crabby posted 08-27-2005 03:43 PM ET (US)     Profile for crabby  Send Email to crabby     
If the battery voltage under load is dropping down to 7 volts at the battery terminals then it sounds as if there is a large amount of high resistance internal to the battery. Possibly one of the plates is bad or maybe one of the internal connectors between the cells has crapped out not enough to stop you from seeing 12 volts under no load but enough such that it cannot handle the draw from the invertor.

A quick rule of thumb when using an invertor: you are stepping up the voltage by a factor of ten, so you are using ten times the amount of current (on the 12 volt side) as the device operating at 120 volts plugged into the invertor is using. So your electric coffee pot is maybe drawing 4 or 5 amps at 120 volts but it is sucking 40 or 50 amps from the battery at 12 volts (which is a substantial draw).

kingfish posted 08-27-2005 04:04 PM ET (US)     Profile for kingfish  Send Email to kingfish     
Thanks crabby - as is not uncommon I guess, as this thing is going along, I'm learning more stuff about batteries (and inverters) than I knew there was to know...

The voltage drop I'm getting at the battery terminals on the gel battery is with no load other than the inverter itself, which I think is in the 0.5 amp category, so the inverter is sucking maybe 5 amps at 12 volts and knocking the terminal voltage down to 7 volts? Sounds like something not good in that battery, huh?


Jerry Townsend posted 08-27-2005 05:54 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
John - my apologies as I didn't read the 'tongue in cheek' and initially thought your question was strange in view of our previous discussions.

Your experience with the gel battery and the inverter is strange and interesting - and can only be caused by the inverter connected to the gel battery. An answer from the battery manufacturer - or other gel battery expert would be very interesting.

From what you say - I would almost bet that the load discharge rate of the gel battery is pretty low and hence it would not work!! as a starting battery. Also of interest would be the voltage on the 'output' side of the inverter when connected to the gel battery. From what is known right now - I would not be surprised if it was around 70 volts.

And John - I believe that they make DC coffee makers - though I am not sure of the 4 cup version. ---Jerry/Idaho

where2 posted 08-27-2005 11:35 PM ET (US)     Profile for where2  Send Email to where2     
Attach a handheld spotlight to the gel cell batteries. If they still drop to 7v under load, the batteries are trashed and should be recycled according to your local regulations.

I have a 36v golf cart running off a combination of wet and AGM cells (not the recommended method, but I got a bargain on the batteries, so I took what I could get and worked with it). You can load the heck of of the AGM when they are new, but over time the AMG exhibit the same failure symptoms as a wet battery. (they charge to 13v, but drop to 10v or less under load).

davej14 posted 08-28-2005 09:18 PM ET (US)     Profile for davej14  Send Email to davej14     
If the gel batteries are not capable of holding up the voltage of your inverter when it is not loaded, and the inverter performs well with your other batteries, then the gel batteries are defective. You can charge a "dead" battery to the recommended voltage because there will be a "surface effect" on the plates that have oxidized. When you apply a load this charge will quickly blead off. I believe this is your situation.
kingfish posted 08-28-2005 10:22 PM ET (US)     Profile for kingfish  Send Email to kingfish     
Thanks all-

Davej14, I've reached that conclusion too. One of the two gel btteries will simply not support the unloaded inverter no matter what, and as it turns out, while it charges to about 12.8 volts, it never allows the battery charger (which has settings for deep-cycle batteries and low amp charge with auto shut-off) to reach full charge and light the green full charge LED. The other gel battery will allow the charger to reach full charge, and will support the unloaded inverter. In fact it will even support the coffee maker plugged into the inverter and turned on. But the voltage of the gel battery drops right down to just above 10 volts as soon as the coffee maker is turned on, and if I leave the volt meter attached, I can watch the voltage (slowly) drop. So I'm sure one battery is bad and I'm pretty sure the other one is too.

Jerry, I'd be money ahead with a 12V coffee maker, that's for sure, although I haven't found a 4-cupper yet either. But then I wouldn't have learned all that I have here, and I may have found out about my batteries being bad in circumstances far less convenient than in my garage. So it's worked out for the best...

Thanks again-


pglein posted 09-07-2005 06:11 PM ET (US)     Profile for pglein  Send Email to pglein     
Sounds like you guys got it figured out. But I had this same issue on my trawler. The batteries charged up to 12+ volts, but as soon as a draw was put on them they died. Replaced them and the problem was solved.
kingfish posted 09-07-2005 07:04 PM ET (US)     Profile for kingfish  Send Email to kingfish     
Thanks pglein-

Apparently I'm not going to hear back from the mfr. about this question, but I have concluded that battery behavior is indicative of a battery that has failed. One battery is shot but I'm still working with the other gel battery that seems to have some potential, but I'm not sure yet how much.


jimh posted 09-07-2005 09:55 PM ET (US)     Profile for jimh  Send Email to jimh     
After 30 years of trying to diagnose problems with battery operated equipment, my first rule is to try a new battery. This tends to solve a huge percentage of the problems. If a new battery does not fix the problem, then there is really something wrong with the equipment or the wiring, but a new battery is the first thing to try.

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