RE: [PSUBS-MAILIST] uneven charging

This is a really interesting thread to me because I'm in the process of building my battery pods. I will have two pods with six batteries in each. My intention is to connect the batteries in each pod in parallel, so that each pod would be 12V. The main propulsion and thrusters would run on 24V by connecting the two pods in series. Smaller loads could be run off either pod individually even while they are both connected in series. I have a charger with three independent 12V outputs, so I can simultaneously charge the two pods plus a single battery that will go in the hull for things such as instrumentation. If all batteries are the same size and brand, bought at the same time, do you think performance unevenness is really a significant issue? The alternative of course is to run the main propulsion motor on 72V, but I would have to use a DC-DC converter to lower the voltage for the trolling motor thrusters, and that's mighty high current for a DC-DC converter. Even if there is one that will handle it, the loss in efficiency might be greater than my losses due to battery unevenness, right? What I'd really appreciate is if anyone has a notion of the magnitude loss of capacity we're talking about due to unevenness. Anyone know?

Thanks,

Alec

The contents of this e-mail are intended for the named addressee only. It contains information that may be confidential. Unless you are the named addressee or an authorized designee, you may not copy or use it, or disclose it to anyone else. If you received it in error please notify us immediately and then destroy it.

From: owner-personal_submersibles@psubs.org [mailto:owner-personal_submersibles@psubs.org] On Behalf Of Jay K. Jeffries

Sent:   Friday, May 09, 2008 6:44 PM
To:     personal_submersibles@psubs.org
Subject:        RE: [PSUBS-MAILIST] uneven charging

Forklifts and large submarines have a luxury that unfortunately we cannot afford in PSUBS and that is the use of large volume batteries. An application closer to us is the scooters used in tech diving that are used to explore miles of underwater caves in a given dive. They test their batteries by measuring the various discharge rates of batteries and group similar discharge rated batteries together as they have the same charging rate and do not display the issues described in previous posts. They are tested regularly over their lifetime to insure matched service. When you purchase batteries, you can request that the battery firm provide you with a matched set for a small premium and thus save buying a bunch of batteries and matching them yourself. If you replace one, you have to replace all to keep matched service.

R/Jay

Respectfully,
Jay K. Jeffries
Andros Is., Bahamas
Vive vt vivas
— Live that you may live

_____________________________________________
From: owner-personal_submersibles@psubs.org [mailto:owner-personal_submersibles@psubs.org] On Behalf Of Hugh Fulton
Sent: Friday, May 09, 2008 18:07
To: personal_submersibles@psubs.org
Subject: RE: [PSUBS-MAILIST] uneven charging

The best solution, I have been told, is to use 2 volt cells in series. This is how forklifts etc do it as it is very hard to get the internal resistance of batteries to be the same. Forklift batteries generally have 2 volt cells that are up to 2000 Amp-hours per cell. This is why you should always recharge batteries, that are in parallel, in isolation. Also batteries left in parallel can discharge themselves through this unevenness. All this was explained to me by a battery importer. Makes sense to me but I am still going for parallel as the 2volt cells with high amp hours are a crazy size physically. This probably explains some of the problems I have in my boat which has 3 batteries in parallel. It is very common advice in the marine industry that id you are using batteries in parallel you should use the same manufacturer, size and model of battery.

Hugh