Ah, I see we have another person who believes the alternator does a good job fully charging a battery, quickly.<img src="/images/boards/smilies/biggrin.gif" alt="" align="absmiddle" border="0" /><br /><br />The opposite is true. <br /><br /> Unless your drive to town and back is 8+ hours, you returned only a tiny portion of the charge back to the battery.<br /><br />While a cold vehicle alternator spinning at high rpm is capable of producing high amperages for a short while, these amperages fall to very low numbers far too quickly. When a second battery is added to the charging circuit over longer thinner cabling, the performance suffers even more. Even a Beefed up charging circuit is still voltage regulated. The vehicle's voltage regulator, whether in the engine computer or the alternator itself, does not want to see more than ~14.5 volts, so the alternator only makes enough amperage to hold 14.5 volts. Even on a depleted battery, this can be as low as 7 amps. So discounting inefficiencies of battery charging lets do some fuzzy math. Your battery needs ~70 amp hours to be in the 95% full range and your alternator, after a half hour, is only sending 7 amps into the battery. 10 hours driving to get in that 95% range. That last 5% can take another 4 hours, and might not ever get to 100% if one were to measure specific gravity of the cells.<br /><br />A better higher rated alternator is not going to significantly decrease battery charge times either, unless you are running 70 amps worth of driving lights or some other huge load on the electric system. Some aftermarket voltage regulators are capable of charging batteries at a better rate, but in general, vehicle's voltage regulators are designed to fall on the safe side, which is undercharging the battery. An overcharged battery boils off more electrolyte, and has more space above the plates for hydrogen to build up. Add a spark and think about the Hindenburg but add some sulfuric acid to boot.<br /><br />So it is pretty much a given that voltage regulators are programmed to undercharge batteries by reducing voltages well before the ideal time for best recharge. While a wall/ grid powered charger can vary the aspects of current, say pump in 25 amps at 14.5 volts, and alternator only produces the minimum to hold the battery at 14.5 volts. If this is 7 amps, them 8 amps would push it upto ~14.8v, outside the parameters set by engineers and bean counters with the lawyers hovering over both of them.<br /><br /><br /><br />Measuring voltage right after driving does not give an accurate diagnosis of state of charge of the battery. You will be reading the surface charge. Your 12.7 volts right after driving, 6 hours later without load on the battery, could easily fall to below 12.0 volts. It takes a long time for surface charge to burn off, even with a load applied. This is why you can jump start a car drive it a mile, shut it off and instantly restart it, then drive for an hour, shut it off for 2 hour and it will not start.<br /><br />Using voltage to determine battery state of charge is only accurate on a rested battery, one which has not seen charging or discharging sources for several hours, and sometimes even a day or more can pass on a healthy battery before surface charge fully dissipates.<br /><br />Wal mart marine batteries are not really very good at cycling very deeply, and in general are not very good batteries, depending on how much cost cutting the manufacturer in your area employed to meet Walmart's standard. The battery Johnson Controls makes for Interstate is NOT the same one they make for Wal- Mart.<br /><br />Whenever any battery sits below 80% charged, the lead sulfates on the plates begin to harden and the battery loses capacity. The lower below 80% and the longer it stays there, the faster the capacity is lost. This capacity loss, in general, is not reversible.<br /><br />A battery that is cycling daily should be brought upto a true 100% at least weekly. This takes a long time to accomplish, at least overnight, even plugged in with a big 12+ amp 'smart' charger. That last 15% is squeezed into the battery only very slowly, whatever the charging source. Voltages need to be brought up into the mid to high 14's and held there for hours.<br /><br />Forget how old/new this battery is, it now has lost a good portion of it's capacity. You can fully charge it via a wall charger overnight, or longer, then you might have 80% of what it had originally, and that is pretty much best case scenario. <br /><br />What basically has happened is you have repeatedly discharged this battery more than what you have returned to it, and now it is gasping for air.<br /><br />You might be better off taking it back to wal mart partially charged and get a new one or pro rated one under warranty if that even applies to the battery you bought. ( surface charge is a good thing while trying to get a battery replaced under warranty. good for you, not the warranty provider) <br /><br />You must think of an aging battery like a fuel tank which keeps getting smaller. You can still fill it up, it just takes less time to do so, and has less to give.<br /><br />I know much of this is not what you want to hear. Sorry.<br /><br />I currently have a digital ammeter that shows alternator amperage and counts amp hours returned to or taken from my batteries. Before I had this very informative tool (and solar) I was replacing alternators yearly and batteries every 8 months. <br /><br />Since I have had this tool, I beefed up my van's charging system, with significantly increased cabling, and a smaller alternator pulley, and basically there was only a small improvement in the time it takes to bring depleted batteries back to the 90% range via the alternator. Above say 70% and all my improvements did nothing, they were only beneficial when the batteries were very low and only for the first half hour of driving, if that. <br /> <br /><br />Ignorance was bliss. <br /><br /> I have come to realize how poor my understanding of alternator charging was, and I still have mechanics or otherwise knowledgeable persons stating just outright falsehoods when it comes to battery charging via alternator. I've argued with some of them till blue in the face, pulled out clamp on ammeters, voltmeters, hydrometers, dead batteries, ect to prove my point but 8 times out of ten I get a:<br /><br />" My new expensive 130 amp alternator will fully charge my new dead battery in an hour" despite lots of contrary evidence proving that that, is simply impossible.<br /><br /><br /><br />
