[font=Verdana, Arial, sans-serif]You make 5 stops and start the car 5 times. Your total driving time is 1 hour with quite a bit of it idling.[/font]
[font=Verdana, Arial, sans-serif]According to the numbers, is that certain death for your battery? It's certainly much worse than using the microwave for 5 minutes[/font]
I've seen my digital ammeter register my starter's draw at 125 amps, but I'll guess this is closer to 175amps actual as the refresh rate of my Ammeter is only a few times per second.
How long does one crank their engine to start it? Mine, a 1989 dodge with a 318, rarely cranks for more than 3/4 second before catching and that is when overnight cold.
The actual energy taken from my battery to start my engine is tiny, a Small fraction of total battery capacity. All modern fuel injected engines are similar. Exceptions are older carb'd engines which might need to crank and crank before the engine catches.
My Ammeter shows that when I start my engine with a fully charged battery, within 2 minutes, the amps required to maintain 14.9v, is back down to the 0.3 range or less. Fully charged on this battery is when the amps required to maintain absorption voltage is less than 0.42. Yes 14.9v at 77f is excessive, and a topic for another thread, or not
So, Less than 2 minutes is required in my vehicle to replace the energy used to start the engine. 14.4absV would only be a few seconds difference in this scenario. Subsequent warm starts that day and the engine catches even faster, often just the merest blip of the starter, a 125 to 175 amp load for a fraction of a second. I could start and stop my engine for hours on end before the battery got too weak to accomplish the task.
Are you actually comparing a Microwave's 120 amp load applied for 5 minutes vs a car starter's 175 amps for a second or less and thinking the microwave comes out ahead? I certainly hope I misread that.
Daily driver cars have no need to ever discharge their engine starting battery below 99.5% charged, as the starter takes practically nothing from it. The fact that they last 5 years in mild climates is not surprising when they are 99% charged or more their whole life. The alternator only then has to make a small amount of electricity more than required to run the vehicles electrics, and is not stressed.
Comparing a starting battery longevity in a daily driver commuter has absolutely no relation whatsoever to a van dweller deep cycling an engine starting battery, or a house battery.
Modern cars that sit undriven for 3 weeks, are however deep cycling their batteries as the door lock, alarm, engine computer is slowly drawing the battery down to ~35% charged at which point a jump start is needed. These batteries can and do fail within a year as even a 4 hour drive will not yield a fully charged battery, and then the car at some point gets to sit for another 3 weeks, and the starting battery deeply cycled again sure to be undercharged again, wash rinse repeat until failure.
And Since a starting battery can still start a daily driver car when seriously degraded, and undercharged, they are usually replaced only when some event pushes the battery too far, whether it is cold weather snap reducing the available capacity of the battery while increasing the starter load, or simply leaving a cell phone charger plugged in overnight, with no cell phone attached. The straw which broke the camels back, not the microwave dropped on its back from the 100' tall palm tree.
Most starting batteries when they are replaced, are incredibly weak, but their task was weak too. A house battery when this degraded and weak would be overwhelmingly obvious to even the most ignorant of battery depleters that something was not right with their battery.
464 AH of relatively healthy fully charged GC batteries, under a 120 amp load, yields a battery bank of only 226.25 amp hours total capacity to give under that load. Half of that for no more than a 50% depletion is 113.125AH.
I am estimating a Peukert number of 1.15 for T-105 batteries, which is likely generous as average is 1.2 for lead acid batteries. 1.1 is the number on the nest AGM batteries which have the lowest internal resistance. Unhealthy flooded batteries can have Peukert numbers of 1.3 or higher. With a 1.3 peukert number that 464 amp hour capacity of GC battery under a 120 amp load yields only 110 AH total available capacity. 55 ah depletion available ( from a 464AH bank) before depletion to 50%.
http://www.csgnetwork.com/batterylifecalc.html
Good thing you do not need to run that microwave for an hour each night.
BUt there is value in warm and fuzzies knowing that you could.
Or can you?
It assumes your batteries are healthy and starting out fully charged. Do you get these batteries upto 14.8v at 77F as Trojan recommends? Do you then HOLD this absorption voltage UNTIL amps taper to 9.28 amps which is 2% of 464?. Did you dip your hydrometer when amps taper to 9.28 at 14.8v and see if all the cells are 1.275 or higher, as this would indicate a healthy fully charged battery? Do you own an ammeter or a Hydrometer?
No? Then you have Absolutely no quantifiable idea if you are fully recharging your batteries, and no idea of their health either, but you can certainly assume
But your solar got them upto 14.3v by one oclock..... How can you read anything about lead acid batteries and think this in anyway indicates a battery anywhere near full charge? I can deplete my battery to 35% charged, and drive for 5 minutes and my battery is at 14.9v, as my alternator can feed it 80+ amps when depleted to this level. Does this mean it is fully charged? Of course not, Not even close, it's going to take 5 more hours of driving at a minimum, before this high quality, low resistance AGM battery, wired with a very thick low resistance charging circuit from a 130 amp alternator, will be anywhere near 100% charged.
Getting a battery upto a voltage means extremely little, without knowing the amps flowing at that voltage. Don't know the amperage? Then you are practically blind.
There is NOTHING magical about the electricity produced from an alternator which charges a battery faster. When pressurized to a certain voltage, the battery can only take so much, and the more it has taken, the less it can then take.
So basically, No, your 4 t-105 batteries could not power your microwave for an Hour at night, as while they still obviously have enough capacity to meet your current needs, they are degraded to some degree as they are aged, and not being fully recharged either, as you can't determine full charge without an amp hour counter, or an Ammeter, or a Hydrometer. Voltage alone means next to nothing. It is a shame when voltage alone is used as proof recharging ability or state of charge as there are so many influencing variables..
Good thing it only takes 5 minutes to microwave a potato. It should be some time yet before those Trojans degrade enough for you to no longer consider them a chest thumping 'just fine' and 'going strong'.
Which is a testament to the quality of Trojans t-105s and their resistance to abuse. Good thing they are only batteries and simple and affordable for YOU to replace, whenever that might be.
The newbie reader who thinks their alternator can easily power a microwave through a big inverter while running their engine, and quickly and easily recharge the battery to full is not likely going to have a 600$ large bank of high quality Trojans to abuse, nor the funds to just battery swap when the battery' no longer takes a charge' or their alternator needs to be replaced from being maxed out when idling and overheating.
I keep imagining some Newb reading this thread with a 3000 watt MSW harborfright inverter with some jumper cables hooked to it from their engine battery powering their 5$ 2000 watt garage sale microwave with some alligator clips out in the middle of nowhere trying to bake a potato, when while revving their engine, their alternator starts smoking, and the engine stalls, and can't be restarted.
Perhaps this just turns into an adventure and a learning experience they can look back on and laugh in their later years at their fundamental misunderstanding of electricity..
But, worst case scenario, they have no later years, as they died from exposure trying to hike out of the middle of nowhere when they smoked their vehicle's electrical system, trying to bake a potato in a microwave because they read it was possible.
