Having done this "band aid" battery reconditioning myself in my college days with no money, I can tell you first hand the results will be a 90 day life max. The first challenge you will discover is what do you drain the battery acid into? A plastic bucket works, then what? Dump it in a county park somewhere?
Rinsing out the battery without losing all your pants and shoes to battery acid damage is the second challenge. The scaling trash from the battery plates that comes out is interesting and it will give you insight into battery construction.

Charging up a battery that is on its last legs is an interesting project, but is was "fully charged" in the vehicle, as the battery in question could only hold a surface charge that was approx 20% of its original capacity. This is why it cranked slowly. Low available amps reserve in the battery.
Bottom line: lots of effort = poor back up battery.

Education time J
Here is the proper way to test a battery.
Charge up for 2-6hrs using a 4-12 amp charger, (if you own a 40 amp commercial charger you can actually do a volt test when charging after 3 mins…over 15.5 volts? Trash it) apply a load of 50% of cold cranking amp ratings (0 degree ratings not the 80 degree rating used to sell you the battery). For most batteries a 150-250 amps load will expose battery state in 15-30 seconds. Hold load draw for 15 seconds while noting battery voltage at the beginning and end of test.

Starting voltage 12.7 or higher
Draw reading should NOT drop below 10 volts
If battery drops below 9.6 volts, throw in trash/recharge and retest, replace if fails again.

All of this can be done in a car or outside on a bench with proper test equipment. For a by the side of the road quickie test, all that is needed is a voltmeter. Read battery voltage with key off >12.0 volts? Snap on headlamps on high for 30 seconds battery voltage >9.6? Start / jump-start vehicle, read voltage at battery terminals >14.7? Charging system is OK. Replace/charge battery.

We will not get into starter draw testing on this posting, as I am sure eyes are glazing over already. We will also not get into battery testing on ones that short out internally, as the reading go everywhere but to a normal range. Ever seen 20 volts on a car battery?
It makes electronics go nuts in our cars. The fun ones are the ones that will not jump when connected to the car as the battery is shorted/dead and consumes all the voltage from the cables. Disconnect the battery cables from the installed battery, jump directly to the battery cables and presto the car starts.

Never run a car without a battery connected as the shock to the regulator will not be good.

Special note to everyone now that winter is here. NEVER jump anyone with the “good” car running. The shock to the electrical system will wipe out something 40% of the time. Just hook up the cables, run the good car without trying to start the dead one. This will charge the battery on the dead one with little chance of damage to either car. Charge for a few minutes and turn off the good car and crank the dead one. Should start right up.

Class over, click next posting….

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2005 X5 4.8IS
The Blue ones are always FASTER....

Current Garage:
2005 X5 4.8is
2002 M5 TiSilver
2003 525iT
1998 528i
Former Garage Stable Highlights
2004 325XiT Sport
1973 De Tomaso Pantera, L Model
1970 Dodge Challenger T/A 4 sp Alpine White
1970 Dodge Challenger T/A 4 sp GoManGo Green
1971 Dart Sport, “Dart Light” package
1969 Road Runner 383
1968 Ply Barracuda 340S FB Sea-foam Green