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Discussion Starter #1
After fishing out of boats for many years I cannot decide which is best. Keep the batteries topped off after every trip even if it only been used for an hour or let the battery drain down and then fully charge.
Any opinions??
 

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depending on the battery, i have read that its not a good idea to keep topping it off each time because it could develope a "memory". Meaning that over time, it only thinks it needs to work for an hour to two and not the typical because it has always been charged up after an hour to two. I run my trolling motor till it dies then recharge it.
 

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Finally, a topic that I feel qualified to answer! In "real life" I'm an electrical engineer...

The term for completely discharging and recharging a battery is called "deep cycling".

Lead Acid type batteries do not like deep cycling. Standard car batteries can be damaged quite a bit by deep cycling. Do it too many times and your battery will stop taking a charge. Leaving your lights on is really bad for most car batteries. Some lead acid batteries are labled as "Deep Cycle", as in "Deep Cycle Marine". These batteries are more difficult to kill by deep cycling, but I wouldn't tempt fate if possible.

NiCD batteries, and to a lesser extent NMHi, prefer to be deep cycled. They have a "memory effect" (which is a misleading name for the problem). Essentially, if you don't deep discharge them then they will tend to loose their ability to keep a charge. _BUT_, and this is a big _BUT_, deep discharging them incorrectly can be worse than not doing it at all. Let's say that you have a flashlight with four NiCD's in it. You "could" deep discharge it by leaving the light on for a day, but this would be bad... What happens is that each battery will have a slightly different charge in it, so as the flashlight burns down one battery will completely discharge before the others. If you work out your electrical theory stuff you can figure that the dead battery will essentially be charged IN REVERSE POLARITY by the remaining batteries. Charging in reverse polarity is a quick way to kill a NiCD or NMHi battery.

For most people, the best way to deep discharge NiCD or NMHi batteries is to "mostly deep discharge" them. That is, run the batteries down but don't let them go all the way down. Most electronic equiptment (cameras, MP3 players, PDA's, GPS's) are not capable of completely draining a battery. But things like motors and lights are able to do that. So for these devices, turn them off when the light is no longer useable or the motor no longer turns with enough power to be useful.

Lithium Ion batteries seem to be the most immune from charge/discharge related issues, but I still wouldn't deep discharge them-- only a mostly deep discharge. Also, most Li Ion equiptment has more sophisticated charge/discharge circuits in them to prevent these issues.

So, in the case of your boat... If your battery is a Lead Acid, for the trolling motor or whatever, then you should keep it topped off.
 

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Some lead acid batteries are labled as "Deep Cycle", as in "Deep Cycle Marine". These batteries are more difficult to kill by deep cycling, but I wouldn't tempt fate if possible.
So are you telling me not to charge on the deep cycle setting?

Dan
 

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Discussion Starter #6
I think he is saying not to leave your deep cycle uncharged. I think any marine battery is deep cycle.
 

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Right. Don't leave any lead acid battery uncharged if you can help it.

But, Dan, you email implies that your charger has a "deep cycle" setting. This could mean many things, so the best thing in that case is to consult the manual. There are NiCD/NMHi chargers that will first discharge the battery then charge it. This is appropriate for those batteries, but not for Lead Acid batteries.
 

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the best way to charge lead acid and agm batteries is with a properly sized and adjusted 3-stage charger (bulk absorbsion and float)

the bulk stage puts out the full amps available from the charger untill a preset voltage is reached then the absorbtion stage holds that voltage and tapers off the amps over a preset time usualy a couple hours then the float stage drops the volts back and hold it there to keep the batteries charged with out over charging.

with this type charger properly sized to you battery bank the life of your batteries can be extended greatly

i like a charger that has an output of 20-25% of the amphour capacity of the battery bank but have used ones with only 10% output with good success. i curently use a 3-stage 50 amp charger with a 400 amp hour bank (4 trojen T-105's)

a full discharge on any lead acid or agm battery is devistating most car and marine batteries dont like mor than a 10-20% discharge and high quality deep cycle (trogen T-105s and L-16s and similar) dont like more than 50% also a 50% discharge does not mean 6volts its more like 11 volts most good inverters have low voltage shut offs that kick in around 11 volts

most if not all of the "marine" batteries are just slightly heavyer duty car batteries to get good true deep cycle batteries 6 volt industrial batteries are the way to go. you dont see electric fork lifts with "marine" batteries

back to the nicads and that realm of batteries what is worse, to recharge a partialy discharged battery or to leave the same partialy discharged battery to sit for some time
 

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[back to the nicads and that realm of batteries what is worse, to recharge a partialy discharged battery or to leave the same partialy discharged battery to sit for some time /quote]

From my two decades of r/c race car experience, I have cycled thousands of $ in batteries and have to consider myself an expert by default. Nicd batteries have become obsolete, they are not going to go away but are considered fragile compared to nimh, Nicd has to be discharged after use for top performance, too far and you get the reverse polarity like DavidK mentioned and it is junk, not enough discharge and it builds "memory" and will loose capacity or run time. Nimh should be stored with charge in them, like at least 1/2 way to fully charged, We will discharge fully charged sub-c nimh cells to .9v per cell then charge but this is not necessary to operate things like digital cameras and gps and it takes specialized equiptment, so for nimh, charge after use, nicd, charge before use
 

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so with the nihm you would recharge right after use even if they were not fully discharged and you wouldnt let the sit with a partial charge? did i get that right?
 
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