I wonder what you think about the following problem:
A car's battery needs to be replaced but the exact capacity battery is not available, though smaller or larger ones are. My feeling is that the battery capacity is matched by the manufacturer with the alternator's charging capacity, so the replacement battery should also closely match the OEM battery's capacity. Is my assumption correct, or it makes not much difference to use somewhat larger capacity replacement battery than the original?
The matching of total battery capacity, to instantaneous maximum charging current available from the alternator, is not as critical as you seem to feel.
Basically, the battery capacity is going to give you a measure of how much starting power you can pull out of the battery (over a prolonged period of cranking) before the voltage starts to drop. Larger is (again, to a first approximation) better.
Once you start the car, the charge control system (alternator and control logic) is going to supply current to recharge the battery. Typically, these systems are designed as (fairly crude) "constant voltage" supplies - the alternator output is allowed to float upwards to higher voltages, but is limited by the control system to no more than around 14.4 volts (nominal). If the battery is heavily discharged, the battery will draw about as much current as the alternator can supply; the alternator's voltage won't reach 14.4 and the limiting won't occur. Once the battery is largely recharged, its terminal voltage will rise and it will draw less current from the alternator, and the charge-control system will prevent the alternator voltage from rising above 14.4 volts.
If you put in a higher-capacity-than-before battery, then (if you don't deep-discharge it) starting the car will draw out a smaller proportion of the battery's stored charge. Its terminal voltage won't drop as much as would be the case in a smaller battery. It'll probably draw just about the same amount of recharge current as a smaller battery would, though, and so the alternator won't really "notice" a difference.
Under conditions of really deep battery discharge, the higher-capacity battery might be able to "take" more current than your present alternator can supply. I don't believe this is harmful, though... the alternator's output voltage will simply drop (due to e.g. resistive loss in the alternator windings) and this will automatically reduce the current into the battery to what the alternator is capable of supplying. The same thing happens with your current battery, if the vehicle is idling and the alternator isn't running very quickly... its output voltage drops, the headlights dim a bit, and everything balances out.
Unless there's something really unusual about your car (I believe), replacing an OEM battery with a new one having even twice the capacity shouldn't present any sort of problem (other than the physical size of the battery).
Actually, alternators are usually sized to the electrical load that the car designers think the car will present -- cars with little electrical equipment can get by with smaller alternators, cars with lots of electrical equipment need bigger alternators. You'll find camper trucks out there with alternators up to 100A (or more: 100A in a 4-wheeled road vehicle is where my brain fuzzes over).
The car has a dingus called a "regulator" in it somewhere (these days it's usually built into the alternator). This holds the voltage from the alternator to a reasonable value for battery charging, regardless of the speed of the alternator, the condition of the battery, or the electrical load.
This, in turn, means that as long as the charging system is in good shape you can have a wide range of battery capacities and be perfectly fine.
The bottom line is: if you get a battery the correct physical size for the car, you should be OK. Even if you go much larger, you should be fine. If you go much smaller the battery may not have enough oomph to start the car, but if you're getting the correct sized battery then that probably won't happen.
It's the Lead washer used between the double leads on a side terminal battery, that causes problems. It thermal cycles and causes a bad connection. Tightening it compresses the washer such that the bolt eventually protrudes into the battery and causes a leak. The bad connection usually causes the diodes in the alternator to go kaput. I replaced mine with a brass washer and never had a problem since.
As for the battery, largest that will fit and the longest warranty available.
Actually the regulator doesn't control the voltage. The altenator is a current source, the regulator controls the current produced by the altenator, increasing the current when the battery is low, reducing it when it is full, the battery keeps the voltage stable.
3,496,447 Alternator Voltage Regulator Responsive to Temperature
3,505,590 Temperature Responsive Output Voltage Apparatus
3,522,482 Temperature Compensated Voltage Regulation
3,546,563 Alternator Voltage Regulation Utilizing A Constant Current Source
| James E.Thompson | mens |
| Analog Innovations | et |
Costco batteries are very cost competitive and have a great warranty. I don't think the batteries are any different though. Mine went out after a bit over three years and it only cost me $10 or so for the replacement. I think the first 3 years the replacement is free, then prorated over the next 6 years. I can't recall if they install batteries or not. I know the install tires of course.
I think you are splitting hairs. I believe the voltage is what is sensed and so what is regulated. Yes, the current is controlled, but the two go hand in hand. The circuit is not high gain like in a power supply in your computer, holding the output voltage very steady. Instead when the voltage droops it pushes more current out to limit the droop, and vice versa when the voltage increases.