Car battery Question?
I’m told, if you connect a very weak battery to a new battery, the batteries will equalize. I’m assuming the new battery will now have 50% of it’s power & 50% of prior volts & weak battery will now have 50% power & 50% more voltage. Is this correct? And how long will it take for this to happen?
Will this damage either battery, and isn’t this very similar to a “fast / quick recharge” shops usually do?
Thanks in advance!
This is basically correct. We’ll assume your “weak” battery is actually a good battery that has been mostly discharged.
Conecting it to a fully charged battery will enable some charge to be transferred from one to the other. The process will be slower than if you used a 120v AC battery charger because the difference in voltage between the two batteries is not very great. When you are finished the two batteries will both have the same voltage (roughly 12.5v DC) and will share the charge. The process does no harm to either battery. They are doing what storage batteries are supposed to do.
this new BBS stinks, where DO the replies go??
sorry for the rant.
I think you are mistaken in your belief that the batteries are “equal.” the juice may “equalize” as you refer to it, but there are other issues which make the equation difficult to ascertain.
In your case, the old battery (since you mentioned in your other post that you replaced an alternator) may have been the culprit. if you are having continued starting/ cranking problems the dead battery may be a further symptom of problems. the short list is: 1. bad battery. 2. bad (corroded) battery cables (to and from the battery to ground, and the positive cable to the starter solenoid. 3. bad starter/solenoid.
the cables actually can corrode inside the plastic covering, so even though you don’t see the corrosion, they can be just about useless. you can feel the cables, and flexing them will show where the inside cable is collapsing is where they are corroded. usually the cables show the green powdery dust around the ends (not to be confused with the white dust around the terminals.)
on to this post:
i think you are misusing a “Boost Battery” thinking it is going to work like a high quality, high capacity “Battery charger.” It AIN’T!
Your idea of equal batteries would only work in a perfect environment.
your old battery may have a dead cell, bad cable, or loose/corroded terminals so you will never get it to take exactly half of the juice, since it can’t take it.
have you gotten the car to start at all since you replaced the alternator?
has the car run for 15 minutes or so, at high RPMs to kick the alternator in, to charge the battery?
when you shut the car off, and let it sit, will it restart? ever?
more info.
Cappy208 has it right. Your assumption about the 50% equalization will only occur in a perfect environment where both batteries are equal and healthy enough to accept a charge.
A fully charged battery has more resistance across the terminals than a partially charged battery. Therefore, if you connect the batteries in parallel, the current will flow from the fully charged battery to the partially charged battery until the resistances of each battery equalize andno more current will flow. However, as has been pointed out, if there is an internal short in the weaker battery, more current will flow from the fully charged battery than is desirable.
As the energy is transfered from the fully charged battery to the weaker battery, the voltage of the fully charged battery drops. With the fast charge units that shops use, the voltage remains constant. As the battery being recharged reaches full voltage, the current flow tapers off because the resistance of the battery has increased.
Here is a safe experiment you can try. Take a two cell flashlight where the batteries are almost depleted and the light is very dim. Replace just one of the cells. You won’t get a much brighter beam, because the weak battery is absorbing power from the fresh battery. Measure the voltage of two fresh batteries and put them in the flashlight. When flashlight becomes dim, remove the batteries and measure the voltage. You’ll find the voltage of both depleted batteries is about the same.
Thanks. 7 month old Battery was load tested and checked out fine. Car would start easily for 1 - 2 days but battery apparently wasnt getting fully charged, 3 - 4 days would not start w/o jump. Problem diagnosis was w/ charging sytem, thus replaced altenator. Car ok now, but just wanted to know info for potential future problems. Still asking how long equalization will take between the 2 batteries? New question…How long will I need to drive car to fully charge a “weakened low” state battery. Some say 1/2 hour, others say hours & hours? I’ve also seen someone say a good day??
Thanks. 7 month old Battery was load tested and checked out fine. Car would start easily for 1 - 2 days but battery apparently wasnt getting fully charged, 3 - 4 days would not start w/o jump. Problem diagnosis was w/ charging sytem, thus replaced altenator. Car ok now, but just wanted to know info for potential future problems. Still asking how long equalization will take between the 2 batteries? New question…How long will I need to drive car to fully charge a “weakened low” state battery. Some say 1/2 hour, others say hours & hours? I’ve also seen someone say a good day??
your last reply is telling.
the battery was load tested, but only lasts a day or two. this info should have been posted on your original post. if you had posted this question first with the previous info you likely would have saved the cost of the alternator!
the problem is not in the battery, or the alternator. it is in the cables, or the starter solenoid. likely you have a parasitic drain (somewhere) which is killing your battery, or the battery just can’t “take” the charge because of cable problems.
if the battery (which is fine) goes dead after two days (like three or five starts,) then the battery isn’t getting charged. So replacing the alternator eliminated that as a problem. having the battery tested (although I personally question walmart as being mechanically qualified to judge this) should have eliminated the battery as suspect. so the problem lies elsewhere.
whats left? cables?, connections?, other added electronics hooked up wrong?, interior lights left on?
what have you hooked up (electrically) recently before the battery had been going dead?
have you had any routine maintenance done recently? starter? fuel pump? install a gps unit? sub woofer? anything electrical?