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Old 15-05-2022, 08:14   #1
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Mixing old and new batteries while charging

I have 2 group 27 AGM batteries as my main bank. (92ah each), and 1 old very abused group 27 AGM as reserve. The 1 reserve battery never fully charges and drains relatively fast. Days are numbered.

I never turn on both battery sources when consuming power, because I understand it can drain the good batteries as the charge attempts to cross level across all 3.

Is that correct?

Secondly, when I motor, I DO turn on both battery banks at the same time which charges both. Is that bad practice?

(When I turn off the engine, I immediately only use battery bank 1).


Any thoughts? Thanks.
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Old 15-05-2022, 08:15   #2
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Re: Mixing old and new batteries while charging

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Originally Posted by ArmySailor View Post
I have 2 group 27 AGM batteries as my main bank. (92ah each), and 1 old very abused group 27 AGM as reserve. The 1 reserve battery never fully charges and drains relatively fast. Days are numbered.

I never turn on both battery sources when consuming power, because I understand it can drain the good batteries as the charge attempts to cross level across all 3.

Is that correct?

Secondly, when I motor, I DO turn on both battery banks at the same time which charges both. Is that bad practice?

(When I turn off the engine, I immediately only use battery bank 1).


Any thoughts? Thanks.
To clarify, my switch is an old school off, 1, both, 2 switch.
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Old 15-05-2022, 08:45   #3
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Re: Mixing old and new batteries while charging

Quote:
Originally Posted by ArmySailor View Post
I have 2 group 27 AGM batteries as my main bank. (92ah each), and 1 old very abused group 27 AGM as reserve. The 1 reserve battery never fully charges and drains relatively fast. Days are numbered.

I never turn on both battery sources when consuming power, because I understand it can drain the good batteries as the charge attempts to cross level across all 3.

Is that correct? yes, the good batteries will try to charge the bad battery

Secondly, when I motor, I DO turn on both battery banks at the same time which charges both. Is that bad practice? depends on how bad the bad battery is and how long you motor, but probably not a problem

(When I turn off the engine, I immediately only use battery bank 1).


Any thoughts? Thanks.
My real thought is that what good is a bad backup battery really and for $100 you could get a good one.
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Old 15-05-2022, 08:47   #4
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Re: Mixing old and new batteries while charging

Nothing wrong with charging both, assuming that the alternator and voltage regulator are sized properly and programmed for it. You don't want an alternator to run at near capacity for long; it will burn up. The voltage regulator, if sufficiently sophisticated, should be programmed for AGM batteries. If it's not adjustable, live with it, but realize that the charging process may not be ideal for your batteries.

Consuming from one at a time protects you from flattening both at the same time, but leaves you with one full and one empty when you start the motor. You might better approach the problem by replacing the isolated battery with a 27 AGM, using both the usual two for the house, and save the isolated battery for starting the engine. I use a separate ordinary alternator for that job.

Think through your scenarios for failure, and have a jumper cable (just the positive side, the negatives are already joined) to start the engine with the house battery, etc.

Good luck with it.
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Old 15-05-2022, 08:53   #5
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Re: Mixing old and new batteries while charging

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Originally Posted by ArmySailor View Post

I never turn on both battery sources when consuming power, because I understand it can drain the good batteries as the charge attempts to cross level across all 3.

Is that correct?

.
Theoretically, yes. As a practical matter, your charging source (alternator?) can’t differentiate among the batteries thereby potentially overcharging the better batteries reducing their lifetime.

Same problem can occur once you replace the bad battery to the detriment of them all.
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Old 15-05-2022, 09:45   #6
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Re: Mixing old and new batteries while charging

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The 1 reserve battery never fully charges and drains relatively fast. Days are numbered.
In my limited experience of group 27 AGM's this means it is toast, end of days, depend on eminent failure.
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Old 15-05-2022, 11:51   #7
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Re: Mixing old and new batteries while charging

If charging passes through an isolator, it is probably a bad idea to charge simultaneously, especially with limited power charging equipment (solar, wind).

For example, if you are charging using solar, the isolator will always primarily feed the bad battery that never seems to rise to the higher voltage. The good batteries may never see a high voltage because the isolator will always route the current to the weak battery. The good batteries will fail sooner due to lack of full charge cycles.

Even with battery combiners, with low charging current sources, the good batteries will constantly be drained of power trying to bring the bad battery up to an equal voltage when combined.

Even with high charging sources like an alternator, things get complicated when charging cycles go below bulk charging.
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Old 15-05-2022, 16:44   #8
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Re: Mixing old and new batteries while charging

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My real thought is that what good is a bad backup battery really and for $100 you could get a good one.
By good battery for $100, I'm assuming you mean traditional lead acid? If that's what you mean I did consider it but didn't want to mix battery types. If I replace the dying one I would like to replace it with AGM so they are all same.
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Old 16-05-2022, 05:48   #9
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Re: Mixing old and new batteries while charging

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Originally Posted by ArmySailor View Post
By good battery for $100, I'm assuming you mean traditional lead acid? If that's what you mean I did consider it but didn't want to mix battery types. If I replace the dying one I would like to replace it with AGM so they are all same.
$115 at Batteries Plus
https://www.batteriesplus.com/produc...-boat/sli27mdp
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Old 16-05-2022, 08:07   #10
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Re: Mixing old and new batteries while charging

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Again, I'd like to stick to all AGM. That link brings me to FLA battery.
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Old 16-05-2022, 08:12   #11
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Re: Mixing old and new batteries while charging

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Again, I'd like to stick to all AGM. That link brings me to FLA battery.
Apologies I copied the wrong link earlier.
I guess you'll be spending a lot more :>)
https://www.batteriesplus.com/produc...at/sli27magmdc
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Old 16-05-2022, 08:15   #12
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Re: Mixing old and new batteries while charging

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Originally Posted by Locquatious View Post
If charging passes through an isolator, it is probably a bad idea to charge simultaneously, especially with limited power charging equipment (solar, wind).



For example, if you are charging using solar, the isolator will always primarily feed the bad battery that never seems to rise to the higher voltage. The good batteries may never see a high voltage because the isolator will always route the current to the weak battery. The good batteries will fail sooner due to lack of full charge cycles.



Even with battery combiners, with low charging current sources, the good batteries will constantly be drained of power trying to bring the bad battery up to an equal voltage when combined.



Even with high charging sources like an alternator, things get complicated when charging cycles go below bulk charging.


Typically end of life LA batteries exhibit a false charge effect , ie the terminal voltage rises quickly with little charge being absorbed , under load the battery terminal voltage falls rapidly.

Therefore the good batteries will not be drained into the bad one.
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