There's really no guessing involved here, as you might suspect. Batteries have optimum maximum and minimum charge specs. Wet cells are 2.2ish volts per cell (it'll be on the charge recommendation sheet or MSDS or something) and minimum is 1.75 per cell(the best I can recall offhand). Meaning, at max charge, a 36volt system ought to be reading about 39-40 volts, and when depleted, will read about 32. These voltages will be "under load". As someone mentioned, volts will creep back up a little if the batteries are left for a bit, as some residual chemical action always takes place, but there isn't much depth to that kind of thing, and the power depletes quickly. Also, with wet cells, there is a risk of reverse polarity issues (pole swapping) once you go below the minimum recommended voltage, especially with strings of batteries and high draw systems like ours. These numbers can be monitored on a simple voltmeter (or a volt-ohm meter in circuit, which is simpler if you don't have something designed in). Just run the batteries down to 32ish, in your case, planning to be done for the day at just about that time, and you should be fine. I'll have to admit, it didn't even occur to me that people were diving WITHOUT some way to monitor batteries!!! It's too much like flying small airplanes without a gas gauge, in my mind, only you don't get to look for an open field to land on.
Vance
-----Original Message-----
From: Dan. H. <jumachine@comcast.net>
To: personal_submersibles@psubs.org
Sent: Mon, 15 Oct 2007 7:58 am
Subject: Re: [PSUBS-MAILIST] battery charge level
Brian, This a VERY good question!
I have a sub and don't have a handle on it. I have a small digital volt meter wired in, so I can monitor either of my three battery systems, one 24V two 36V. The voltage varies quite a lot depending on the usage. After using it and when there is no current draw, the voltage keeps climbing back up, and what it go up to seems to depend on how long the load had been turned off. I don't find much stability in the voltage. When the batteries are almost totally depleted, there is a noticeable reduction in prop speed, but by then there isn't much left.
Just to be safe, my sub is wired where I can switch between the two 36V pods. They are both used during a dive but on always runs out before the other. When either runs out, it's time to quit for the day.
The same problem when charging. It's a guessing game when the batteries are totally charged.
If anyone has a way to monitor, I'd be grateful to here it.
Thanks, Dan H.
----- Original Message ----- From: "Brian Cox" <
ojaivalleybeefarm@dslextreme.com>
To: "Personal_Submersibles@Psubs. Org" <
personal_submersibles@psubs.org>
Sent: Monday, October 15, 2007 1:28 AM
Subject: [PSUBS-MAILIST] battery charge level
> Ok, this is a stupid question but it has been bugging me.
> How do you know how much charge you have in your batteries? > Other
> than keeping track of how long you have run your motors how can you tell.
> I know that the wwII subs would have to check the battery acid levels to
> accurately know how much power was in the batteries. But with the sealed
> AGM or gel batteries you can't measure the acid, and chances are you
> wouldn't be able to anyway in a small sub.
>
> Thanks,
>
> Brian Cox
>
>
>
>
>
>
>
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