Riddle for the electrical guru's.......

[SargeW]

I have noted recently that my chassis batteries, while parked with no obvious loads on them will cycle between 13.8 volts (being charged) to 12.7 volts (full charge) down to 12.3 volts (when the charging cycle kicks in again. The starting batteries are 2- 12 volt heavy duty starting batteries, sealed wet cell, rated at 950 CCA.

I am trying to figure out how much of a load it would take to pull down two batteries of that size. This cycle will happen at least 3-4 times a day that I know of.

Because they are starting  batteries, there is no amp hour rating on them that I could find. Anyone know how to figure this out?

 

[John From Detroit]

Sounds like the charging circuit is turning on and off to me. 950 CCA ....

My jump start battery is 950 CCA. it's only about 35-30 amp hours.. CCA is meaningless when you are trying to calculate capacity.

 

[Gary RV_Wizard]

Trying to figure an amp-hour number from the CCA is nigh impossible and not really helpful anyway.  The real question is the amp draw on the batteries while parked and that can be measured with an ammeter. 


You didn't mention how long it takes for them to drop down to 12.3v, but that is a very substantial drop. If the time is measured in minutes rather than hours, you must have some fairly large power consumer wired to the chassis batteries.

 

[Optimistic Paranoid]

With the exact make and model of the battery, it is often possible to find any missing info on the battery maker's web site.

Do I understand that you are using engine starting batteries as your house batteries?

Cause while Marine/RV batteries might give you 400 cycles at 50% and TRUE deep cycle batteries might give you 1,000 to1,200 cycles, engine starting batteries are usually only good for 40 to 50 cycles at a 50% discharge rate.  It does sound like the converter is cycling on and off in an effort to charge nearly dead batteries.

 

[kdbgoat]

It's because he's rocking out the campground with that Pioneer X6800 with the D8600 amps! ;D

 

[SargeW]

No these are strictly chassis batteries. I have six 6 volt house batteries that are separate. The batteries are labeled as Duracell Ultra Pro, Fleet and Heavy duty, Group 31S, Part number SLI31SA.  I had went to their web site, but the specific information wasn't listed. 

There are 4 different computer boards that are powered by the chassis batteries, which blink happily away at random.  It just seems like a lot of draw, even though this coach has an amazing amount of electronics on board.

 

[Kevin Means]

Marty, just curious... how long does the voltage remain at 12.3 volts? Are the voltage swings gradual, or do they take place rapidly?

Kev

 

[Lou Schneider]

There isn't a direct conversion between CCA and amp-hours, because they require different battery construction to achieve.

CCA measures the peak starting power (Cold Cranking Amps) and batteries that are rated in CCA have thin plates with maximum surface area exposed to the electrolyte, so they can deliver lots of power for a short period of time.  Slow, deep discharges will quickly reduce their working capacity.

Deep cycle batteries have thicker plates with less total surface area.  As their name implies, they're designed to deliver maximum power more slowly, at lower currents over longer periods of time.  Their thick plates can be more deeply discharged without damage, but they can heat and buckle if called on to deliver lots of current rapidly.

That said, a Group 31 marine battery (a compromise between a starting battery and a deep cycle) is rated at 650 CCA or 105 AH deep cycle.  With 1/3 more CCA designed into your starting battery, I'd guess it would have at least that percentage less long cycle capacity, i.e. it is probably equivalent to a 50 - 60 AH deep cycle battery

 

[SargeW]

Marty, just curious... how long does the voltage remain at 12.3 volts? Are the voltage swings gradual, or do they take place rapidly?
Kev

12.3 volts is the trigger for the Spyder system to send a recharge cycle to the chassis batteries. It will usually creep down slowly from 12.7 volts, until it hits 12.3 volts. Then the Spyder system kicks in and if I am watching the Spyder display it will jump from 12.3 to 13.8 volts, indicating a charge cycle.  I haven't had the tenacity to watch it long enough to determine just how long it takes to get from 12.3 to 12.7 volts though.  It does repeat this process a couple times a day at least though.

That said, a Group 31 marine battery (a compromise between a starting battery and a deep cycle) is rated at 650 CCA or 105 AH deep cycle.  With 1/3 more CCA designed into your starting battery, I'd guess it would have at least that percentage less long cycle capacity, i.e. it is probably equivalent to a 50 - 60 AH deep cycle battery

Thanks for the info Lou. And I have 2 of them wired in parallel. So with 12 volt batteries, does that mean there is 100-120 AH available?

 

[John From Detroit]

DEKA Group 31's are 100 amp hours. I think Interstate ranks theirs at 130 (C/20 rate in both cases)

Deka battery specification chart.. Lots of them spelled out

Download that Page (it is a PDF so you can save it after viewing).

 

[Lou Schneider]

That's true for the AGM batteries.  An AGM battery can function either as a starting battery or as a deep cycle, they don't have to be designed as one or the other like wet cells.  I missed that Marty had AGM starting batteries when I wrote the reply.

Thanks for the link to the Deka page, John.  That's a keeper.

 

[cerd]

Get yourself a battery analyzer like the one in the link below. If your batteries are good (90% or higher) you are losing power somewhere.

http://a.co/d/4OYwaM3

On mine, while driving, my engine and single house battery are connected via a solenoid and charge at 13.5V (just below what most would consider failing). When I disconnect the solenoid, the engine battery jumps to 13.8V (low side of normal, but passing). I think the difference is that the house battery has to go 25-30 feet through an old 4AWG wire in order to reach the alternator.

I think you either have a draw somewhere or you have a poor connection acting as an additional load. Take out your multimeter and check your connections. Also, use a clamp style meter to see how much is actually drawing. My engine battery only powers my engine and one deep cycle battery lasts me all weekend since we don't watch TV, fridge runs on propane, and I switched every single light source inside to LED. If we need the furnace, I will have to run the engine for an hour or so to charge the battery, but we rarely are out that late in the year.

 

[SargeW]

Nope, I don't have AGM's, they are sealed wet cell batteries from West Penn manufacturing. And when I threw the disconnect switch and let them sit for a day with no load, they did great.  There is definitely a load pulling them down. I am now trying to determine how much of a load, and is it normal for this Powerglide chassis. 

I have a call out to the chassis shop at Tiffin to speak to their head guy for his input.  This is an "all electric" coach with lots of electronic gizmos (technical term) so the norm for this chassis may be a lot different than an older rig. That's what I hope to find out.

 

[Seilerbird]

Marty - What I would do is turn off all the electric in the coach and then turn one thing on and see if that is the problem. If not turn it off and turn something else on. By turning it on I mean either flip the breaker or insert the fuse. Leave the device off.

 

[cerd]

When finding out where this draw is coming from, a clamp meter lead with a DMM would be most beneficial. You can pull fuses or flip breakers one at a time while viewing the current draw remotely. A clamp meter will show the value, but the display is on the unit, making it more difficult to see. You have to change something, then go look and keep repeating that. If you have a clamp with banana leads meant for a meter, you have a few more feet to move the meter for easy viewing, while keeping the clamp around the battery wires.

This is the type of clamp I am referring to. This one has banana leads that plug into any multimeter. Using one of these requires a little math since they convert the current to a voltage measurement, but its typically just moving the decimal.
http://a.co/d/b44xfGB

 

[Lou Schneider]

Note that the clamp on meter suggested above is for AC, not DC measurements.  There are a few DC clamp-on meters that use Hall Effect sensing, but they're most useful for measuring large currents like a starter motor's draw.  At low currents the earth's magnetic field becomes a factor and even something as small as moving a wire or pulling a fuse that changes the length of wire connected to the circuit can affect the measurement.

A better choice would be to lift one of the battery wires and put a multimeter set for current measurement with a 5-10 amp fuse in series with it between the battery and the removed wire.  This will let you step down the meter range if needed until you can resolve the leakage, while the fuse protects the meter from damage if a major draw should kick in.

 

[cerd]

Also note that I said this was the "the type of clamp I am referring to". There are many brands and models that are AC/DC compatible for as low as $50.

They work fine for smaller loads and bigger loads. I have used them for loads up to 50A. There isn't much over that range that I have had to test. I have found that they don't work for testing alternator current at the battery. There is too much current going in and out to get an accurate reading. For this kind of test, it would work fine. 

I, personally, do not use the in line style current meter because I hate changing fuses and you never know what kind of a draw this kind of drain has. That's why I use either the clamp style meters or a shunt.

 

[SargeW]

Great info guys. One way or another I am going to figure out where my draw is coming from. I will report back when I know something.