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Old 09-11-2020, 10:29 AM   #21
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I rotate which car I drive for errands. Did get a new battery for the old Camry, which we call the "mule".

And I keep jumper cables handy.
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Old 09-12-2020, 02:47 PM   #22
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Quote:
Originally Posted by ERD50 View Post
...Some notes on Battery Voltage vs capacity (measured after battery has 'rested', off the charger, for > 20 minutes):

12.70V ~ 100% capacity
12.48V ~ 75%
12.24V ~ 50%
12.06V ~ 25%
11.89V ~ 0%

Dropping below ~ 10.2V can permanently damage the battery.

-ERD50
Very good information, thanks for posting ERD50. I checked our Toyota RAV4, which has not been driven in about a week and a half. Used my 1985 vintage Radio Shack digital volt meter. Got 12.35 volts. I then hooked up a 1970s vintage Montgomery Wards 10 amp/2 amp battery charger on the 2 amp setting and let it charge for a couple hours. Disconnected the charger, waited half an hour and measured battery voltage again. Got 12.75 volts. Then turned the interior light on for a few seconds and then off and checked again and got 12.67 volts (I read somewhere long ago you can check battery state after charging by measuring no load voltage, but need to first provide a slight current draw to remove the surface charge after charging).
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Old 09-17-2021, 09:56 AM   #23
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Somewhat of a co-incidence, but I started thinking about our car batteries again, and this post is just over a year old, so a good time for an update/reminder.

Since we moved this spring, we can do a lot of our shopping at the very nearby center, just 0.9~1.1 miles away. So that's a high % of very short trips, DW will sometimes walk if the weather is nice and she only needs a few things.

I checked mine (which is in the garage) by opening the hood the night before and letting the battery 'rest'. In the AM, I got ~ 12.38V, so maybe ~ 60% charge. Not bad, but not great. DW's was a little lower, and I didn't give a full rest, since she's parked outside until we get the garage more organized.

Put on the ancient charger I inherited from my FIL, it settles down to ~ 1~2 amps after a while. I left it on for about 8 hours and got to roughly 12.60 the next AM after the 'rest', which is ~ 90%. I also charged DWs for about 5 hours, I need to check and probably add some charge time.

REMINDER: If your battery is low, just "taking it for a long drive" isn't going to do much. It takes a LONG time to bring a battery back up ( I gave a rough estimate of ~ 30 hours from 0% to 100% in the OP).

However, a longer trip every week will probably help to replenish what was drained from starting and the phantom drain when parked. I'm seeing ~ 75mA mentioned at typical for modern cars. So a week of sitting would be 12.6 AH, and my battery is ~ 35AH based on the RC25 rating (but would be higher for small drains), but that's still maybe 25% discharge in a week? If the battery is accepting 2A average while charging (I see it drop from ~ 6A to ~1.2A over the course of a few minutes), that's still ~4~5 hours of driving a week to keep it charged (not counting starting current, but 2 seconds at 200A is very small in amp-hours).

These are very rough, first approximation numbers. A very low battery will also draw more charge current for a longer time, so that offsets all this somewhat. And can get you to a charge level that won't leave you stranded, unless it's in cold weather. I'd guess that even a 25% battery will still turn over an engine OK under good conditions.

Bottom line, I plan to check and probably charge up our cars about every 3 months.


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Originally Posted by NateW View Post
Very good information, thanks for posting ERD50. I checked our Toyota RAV4, which has not been driven in about a week and a half. Used my 1985 vintage Radio Shack digital volt meter. Got 12.35 volts. I then hooked up a 1970s vintage Montgomery Wards 10 amp/2 amp battery charger on the 2 amp setting and let it charge for a couple hours. Disconnected the charger, waited half an hour and measured battery voltage again. Got 12.75 volts. Then turned the interior light on for a few seconds and then off and checked again and got 12.67 volts (I read somewhere long ago you can check battery state after charging by measuring no load voltage, but need to first provide a slight current draw to remove the surface charge after charging).
I've read anything from a couple hours to 24 hours rest. And even the 75mA drain could pull off some of that charge. But I think for general checking, something like what you did is probably close enough to remove that surface charge and get a usable reading. We're not trying to get down to exact %, just ballpark. I may experiment with this, and see if a couple seconds of headlights on and a 10 minute rest is as good as an overnight rest.

-ERD50
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Old 09-17-2021, 12:04 PM   #24
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^ Wouldn't the amount of time to recharge a battery by driving be dependent on the alternator, not the charger? That is, your 30 hour estimate is based on an external charger (I think), but recharging on a long drive would be based on the alternator...?

Just reading along for interest, mostly. I mostly can't grok electricity stuff.
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Old 09-17-2021, 12:19 PM   #25
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The battery on our Honda was 10 plus years old when I replaced it last year.

For the past 8 years we have spent 3 winter months travelling. I have routinely pulled the battery out rather than have it sit in an unheated garage. And changed the insurance to fire and theft only.

Do the same with our summer car when we take it off the road in the fall.
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Old 09-17-2021, 12:31 PM   #26
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I replaced my car's battery about a year ago. But leaving idle for about 3 weeks (staying home, not going out) let the battery die when I needed to go out.

Good thing I have one of those lithium type jumpers.

Now I make a habit to at least drive the car around the block once a week to keep the battery from getting drained. Better safe than sorry.
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Old 09-17-2021, 12:48 PM   #27
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We haven't seen any problems with our Toyota hybrid 12v battery after more than 4 years. We drive mostly trips of less than 15 miles. Of course the hybrid doesn't use the 12v battery to start the ICE. The 12v battery is used to power accessories while the car is off, of course, but I'm guessing that even short trips every day or so can recharge that.

I wonder if I can check the voltage at the cigarette lighter?
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Old 09-17-2021, 01:51 PM   #28
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Originally Posted by SecondCor521 View Post
^ Wouldn't the amount of time to recharge a battery by driving be dependent on the alternator, not the charger? That is, your 30 hour estimate is based on an external charger (I think), but recharging on a long drive would be based on the alternator...?

Just reading along for interest, mostly. I mostly can't grok electricity stuff.
No, not so much.

Your alternator will boost your car voltage to a relatively fixed ~ 13.8V~14.2V. A basic charger does pretty much the same thing.

At that voltage, your battery will accept/draw some current depending on how low the charge is. A low charge and it will accept/draw more current. As the battery becomes more fully charged, the amount of current the battery will accept/draw at that voltage will drop off.

The alternator provides the voltage. The current into the battery at that voltage is determined by the battery, not the alternator.

Think of it this way - if you plug a night light into the 120V outlet, it only draws ~ 4 watts (~ 33 milliamps). If you plug a 500 watt flood light into the same outlet, it draws 500 watts ( ~ 4.2 Amps). It isn't the source that sets the current draw, it is the load.

However, the car alternator can deliver more current (I think ~ 60 amp is typical?) than most home chargers (10 amp limit?), so when the battery is very low, that alternator is probably hitting the battery harder/faster than the home charger. But I'd guestimate that by the time the battery gets to ~ 25% charge, the home charger is not the limit.

For example, with my ~ 60% charged battery, that home charger will initially supply ~ 6 amps to that battery, but within a minute, that's dropped to 4 amps, and within maybe 2 minutes to 2 amps, and settles out around 1.2 amps after an hour (rough numbers). So the alternator advantage is pretty short lived, I'd think.

I may look around, there must be some graphs of current at fixed voltage for batteries of X amp-hours versus their state of charge. Though to complicate things a bit, many chargers do a set current until the battery reaches a certain voltage, and then let the current drop at that max voltage. And there are muti-stage chargers to try to treat the battery well (avoid sulfation, etc), and have several stages programmed for various parameters. I don't think cars have this, they just let the alternator provide that set voltage and the battery does what batteries do.

-ERD50
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Old 09-17-2021, 03:31 PM   #29
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A Battery Tender Jr., or similar battery maintainer, can be a good idea if you don't drive your car regularly. $25 at Walmart or Amazon.
+1

For long idle periods, a battery tender is what you need. It puts out a maximum voltage of 13.8V, and this is the float voltage that can be applied to the battery indefinitely.

A charger usually puts out a higher voltage, and will cause a current to flow into a battery already full. The battery will gas up, meaning converting water in the electrolyte to H2 and O2, and drying up the battery.

A battery tender is usually limited in current capacity. It only needs to supply enough to overcome the phantom drain of the electronics in the car, plus the self-discharge of the battery. In short, a current capacity of just a couple of 100mA, and a max voltage of 13.8V will get the job done.

A battery tender can be used to charge a low battery, but it takes a long time. However, it can be left attached to the battery indefinitely. A battery charger may ruin the battery if left on for too long.

A battery tender is cheap enough. Just get one!
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Old 09-17-2021, 03:36 PM   #30
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Originally Posted by ERD50 View Post
No, not so much.

Your alternator will boost your car voltage to a relatively fixed ~ 13.8V~14.2V. A basic charger does pretty much the same thing.

At that voltage, your battery will accept/draw some current depending on how low the charge is. A low charge and it will accept/draw more current. As the battery becomes more fully charged, the amount of current the battery will accept/draw at that voltage will drop off.

The alternator provides the voltage. The current into the battery at that voltage is determined by the battery, not the alternator.

Think of it this way - if you plug a night light into the 120V outlet, it only draws ~ 4 watts (~ 33 milliamps). If you plug a 500 watt flood light into the same outlet, it draws 500 watts ( ~ 4.2 Amps). It isn't the source that sets the current draw, it is the load.

However, the car alternator can deliver more current (I think ~ 60 amp is typical?) than most home chargers (10 amp limit?), so when the battery is very low, that alternator is probably hitting the battery harder/faster than the home charger. But I'd guestimate that by the time the battery gets to ~ 25% charge, the home charger is not the limit.

For example, with my ~ 60% charged battery, that home charger will initially supply ~ 6 amps to that battery, but within a minute, that's dropped to 4 amps, and within maybe 2 minutes to 2 amps, and settles out around 1.2 amps after an hour (rough numbers). So the alternator advantage is pretty short lived, I'd think.

I may look around, there must be some graphs of current at fixed voltage for batteries of X amp-hours versus their state of charge. Though to complicate things a bit, many chargers do a set current until the battery reaches a certain voltage, and then let the current drop at that max voltage. And there are muti-stage chargers to try to treat the battery well (avoid sulfation, etc), and have several stages programmed for various parameters. I don't think cars have this, they just let the alternator provide that set voltage and the battery does what batteries do.

-ERD50
+1

Lead-acid batteries take a long time to get fully charged. Lithium batteries on the other hand soak up charges like a sponge absorbing water.

To repeat: it takes longer than a drive of 30 min. to recharge a depleted battery. And it does not matter whether you rev up the engine or to run it at idle: the alternator is only putting out so high a voltage, and it takes time for the lead-acid battery to accept the charge by sipping.
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Old 09-17-2021, 03:55 PM   #31
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Yes on the battery maintainer. Attach the leads to the battery and put the plug in connector in the grill area, you don't even need to open the hood to plug in or unplug.
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Old 09-28-2021, 11:45 AM   #32
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Somewhat of a co-incidence, but I started thinking about our car batteries again, and this post is just over a year old, so a good time for an update/reminder. ...
Another update, 10 days later.

Battery was dead, dead, dead in my car on Sunday. Barely a click-click-click of the starter and dim lights. Put the charger on, and took DWs car out for the day.

Didn't want to leave the (ancient 'dumb') charger on overnight, so got ~ 8 hours in, but in the AM after a 'rest' it only read ~ 10.7V - not good.

I finally traced the problem to my driver side rear door not being fully closed when I took some stuff out, and the dome light was probably on for two days. I really thought newer cars would protect against this, and shut that stuff off before draining the battery. Apparently, that drain triggered a cell to go bad.

Oh well, this coming winter would be winter #6 for this (original) battery, so I was planning on replacing it anyhow, so no real loss. Actually, maybe a good thing, it was a nice fall day, so replacing the battery was not much of a chore (but they seem to be getting heavier, the older I get!), and not all that inconvenient in terms of timing.

Since I had the cables loose, I decided to check the standby drain. I have to do this carefully, to not exceed the 10A limit on my meter. Sometimes a fan will run, or the lights come on to "light your way" for 30 seconds or so. So I set up a jumper wire/clip so the meter was not connected until I could see/hear things settle down. Then I connected the meter and removed the jumper. I was surprised to see it was still at 2 amps! Another minute, and it dropped to 1A (still high!), but then 0.58A, then 0.12A, then finally (maybe 3 minutes?), I see it drop to 0.10, then quickly drop further to 0.008A (8 ma). That's a pretty low drain. I set the meter to MAX hold and waited over 10 minutes, and the peak was just 9 ma.

Car started right up, but the battery wasn't at full charge off the shelf (~ 12.37V ~ 70%). I put the charger on for ~ 4 hours, then measured the next AM after that rest, and it was ~ 12.65V so ~95%, so that's good.

Then I got curious about what NW-Bound said about reviving a bad cell by knocking the short loose. So I carefully lifted one end up ~ 1" and dropped it, and did that for each end a dozen times with no improvement. Wasn't interested enough to try to be more aggressive, a cracked case and spilled acid wasn't something I wanted to deal with. But, in an emergency, that approach might come in handy.

One more thing - someone (this thread or another) was saying the start-stop feature (optional on mine, I don't have it) is hard on batteries/starters. Well, I figure the starter is designed for it. But in looking for a new battery, I learned that the start-stop feature requires a heavier duty battery, which was at least $100 more. I wonder if the gas savings even reaches that level, cars just don't use much gas when idling at a stop light, and the lights aren't all that long. And if you are stuck in traffic, you can do it manually.

Oh well, all's well that ends well.

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Old 09-28-2021, 12:14 PM   #33
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Car started right up, but the battery wasn't at full charge off the shelf (~ 12.37V ~ 70%). I put the charger on for ~ 4 hours, then measured the next AM after that rest, and it was ~ 12.65V so ~95%, so that's good.

Then I got curious about what NW-Bound said about reviving a bad cell by knocking the short loose. So I carefully lifted one end up ~ 1" and dropped it, and did that for each end a dozen times with no improvement...
If the battery voltage is at 12.65V, you don't have a shorted cell. If it is less than 11V while being connected to a charger then it is most likely a cell is shorted.

To verify if the "shock treatment" works, you need to check the battery voltage while it is being charged. Once the shorting bridge caused by the gunk at the bottom is broken, you see a distinctive jump up in the battery voltage. However, it will not be lasting, as that bad cell will be shorted out again.

By the way, 12.65V is still poor. You don't want to stop charging until the battery voltage reaches 13.8V. Once the charger is removed, the voltage will settle back down.
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Old 09-28-2021, 12:17 PM   #34
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If the battery voltage is at 12.65V, you don't have a shorted cell. If it is less than 11V while being connected to a charger then it is most likely a cell is shorted. ....
Sorry, I guess I wasn't clear. The 12.65 was with the new battery.

Quote:
By the way, 12.65V is still poor. You don't want to stop charging until the battery voltage reaches 13.8V. Once the charger is removed, the voltage will settle back down.
12.65V was after a rest, not with the charger on.

-ERD50
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