battery voltage drop

Randy Webb · 08-25-2003, 09:15 AM

once a battery is charged to 13+ volts, how much voltage drop should there be after 1 week (with no charging but no use either -- just sitting on a shelf)?

Rot 911 · 08-25-2003, 09:22 AM

Around 12.3-12.6 volts

RoninLB · 08-25-2003, 11:25 AM

after charging I put the lights on for a few seconds to remove the "surface charge".. and then do a reading.........Ron

911pcars · 08-25-2003, 11:33 AM

There might be other variables that will affect the amount of voltage drop, including:

Battery quality and type (wet-cell, gel, sealed, etc.)
Humidity
Temperature
State of charge
Battery capacity
Physical storage conditions (on concrete floor? not good)

Sherwood Lee
http://members.rennlist.org/911pcars

rattlsnak · rattlsnak's Garage

If you're wondering if you have to much voltage drop, the formula is simple. Reserve capacity of the battery divided by 4. That is the most a battery can take over 30 days and not discharge.

example... Battery R.C.=100 100/4=25, or 25 milliamps.
Disconnect the negative battery terminal, put an amp meter in SERIES with the lead you just took off and the neg battery post to get your reading.

Randy Webb · 08-25-2003, 08:55 PM

I'm not following this. 1. How do you determine RC?

2. The battery is sitting on a shelf, so nothing is in series with it (except the capacitive "air discharge" thru the EMF in the air).

911pcars · 08-25-2003, 09:01 PM

Randy,
Just curious. Are you conducting some research test or just curious? If you're concerned with discharging through normal shelf life, you could simply hook it up to a battery maintainer or trickle charger. On the car, you could install a master switch in series with the battery cable.

Sherwood

RoninLB · 08-25-2003, 10:15 PM

Hey Randy
what's important, IMO, is the max millamp draw when installed. a decent number is 60 millamps.. at 100 I start searching. a shop can test your battery condition..........Ron

Randy Webb · 08-26-2003, 09:08 AM

Hi Sherwood, I'm trying to figure out if I should just give up on this battery. It loses voltage pretty quickly after being on the maintainer or being charged. It's a hassle to replace because of the alarm system. I didn't install it, don't know where it is and don't know how to reprogram it if it loses voltage completely. I'd have to tear the car apart to find the components.

RoninLB · 08-26-2003, 09:11 AM

Quote:

Originally posted by Randy Webb
because of the alarm system.

maybe you can jump the new battery in parallel for the install

911pcars · 08-26-2003, 10:06 AM

If I'm reading this correctly, you don't want to lose battery power to the alarm system for fear of losing any volatile memory for alarm features? If so, just temporarily connect another 12 volt battery to the system in parallel like RoninLB suggests, then disconnect the questionable, main battery. Any small capacity 12 volt battery will support a car accessory (lawn mower, motorcycle or loaner battery)

If you suspect the alarm system is excessively draining the battery, connect an ammeter in series with the battery and pos. cable as described and observe the parasitic current when the car is at rest. If the alarm is drawing too much current, replace it with another, friendlier system.

You can perform a somewhat accurate battery discharge test on the car. Trickle-charge the battery for a day or so until it comes up to full charge (use a hydrometer to check). With a voltmeter connected to the battery, disconnect and ground the coil lead, then crank the engine for approx. 20 seconds. Observe the VM. During this time, the voltage should stay above 9 volts. If below, the battery is probably toast. You can confirm with another test at a reputable electrical/battery shop. You want to make sure the battery is NG before dumping it. Many batteries are discarded just because they're discharged but still okay.

Hope this helps,
Sherwood

Randy Webb · 08-26-2003, 04:54 PM

Thx - I know about connecting another battery in parallel when doing this.