so my alternator is shot and drained my battery last night. i put the battery on the charger and it will have about a half charge and is rated with a 90min reserve capacity before i need to leave. think it will make it a couple miles down the road to the shop, maybe 5 minutes with all unnecessary electronics shut off?
I am assuming that your battery charge can charge at a current of at least 20 amps. If so, 2 hours should give you 40 amp-hours of charge. That should be enough to start the engine and run it for an hour. I am assuming that the fuel pump, alternator field, igntion system, transmission solenoids, emmissions solenoids, and ecm require less than 30 amps total.
I would risk it.
Your going to use a lot of the charge the battery has to operate the starter.
make, model, model year ?
Todays cars are very power-hungry…I would not count on more than 15 minutes of run-time…This assumes a battery with at least a 2/3 charge…Forget the “Reserve Capacity” nonsense …
X2 Caddyman. In addition, it will be hard to know the 50% charge level. The charge level reading on the battery charger ammeter is not accurate for that purpose.
I’d leave the charger on it until the you get the engine started. Once it’s running remove the charger and make haste to the shop. I’d also leave the radio and AC (more specifically the blower fan) off.
We purchased a new 1991 Jeep Cherokee. For 4 years it started and ran perfectly. One morning it cranked but would not fire. Since this malfunction happened so suddenly I assumed it was a computer/fuel injection problem that I have no diagnostic knowledge of. I had towing insurance so I had it transported to our trusted shop. They checked battery voltage which was slightly over 11 volts. I was informed a module (ignition)? would not operate at that low voltage. I had the battery replaced and charging system checked. It started perfectly for the remaining 6 years. I don’t know if a partially charged battery will allow your vehicle to start. Also if so equipped be sure the headlight switch is not in the auto position.
"… have about a half charge and is rated with a 90min reserve capacity …“
So I’d guess it has at least ½ (50%) x 90 min or 45 min run time left, probably more (RC is the time required to dischrage a fully charged battery at a 25 A discharge rate; a running engine with all accessories off draws less than that).
”… battery voltage which was slightly over 11 volts. I was informed a module (ignition)? would not operate at that low voltage.“
At 11 V the battery is essentially dead. At 12 V the battery has only about 25% of full capacity.
”… it will be hard to know the 50% charge level."
Got a hydrometer? a digital voltmeter? It’s easy to measure the percentage of full charge remaining.
You can measure the potential with a DVM but not the capacity. Using the pail analogy, two batteries may be 12 inches tall but one may only an inch in diameter compared to the good one that is a foot…
That battery probably has been operating in a low charge state for a while and now significantly discharged. It wouldn’t be far fetched to be compromised at this point. Can’t assume it can be charged to the same capacity as new.