Charging dinghy from mothership

[sparrowhawk1]

What about peukert's law? Lower rates of charge and discharge are just more "efficient"

According to this chart I need to put 44 amps back in. Or am I missing something?


I know from personal experience if I take 30 amps out of my 90 AMP hour battery for 1 hour it will be flat as in barely enough energy to push my boat. I've only done that once.

[john61ct]

Originally Posted by sparrowhawk1

I know from personal experience if I take 30 amps out of my 90 AMP hour battery for 1 hour it will be flat as in barely enough energy to push my boat. I've only done that once

A battery that used to be rated at 90AH capacity (not amps) can degrade in just a few months to only hold a fraction of that if not well maintained.

Your current level of understanding and imprecision of the gear involved make Peukert a moot point for now.

I don't know how your previous question relates to your interpretation of that chart.

As I said, getting just 20A into the target bank may involve draining a lot more than that from the source bank.

For the umpteenth time, if you have **any** concern about energy efficiency, all batteries should only be refilled from an actual charge source, not by draining stored power from one bank to another.

[john61ct]

a relevant post from another parallel thread

http://www.cruisersforum.com/forums/....php?p=2778485

[sparrowhawk1]

The battery is new. The chart shows that I need an 88 amp hour battery to use only 50% with 30 amps at 1/2 hour. 50% of 88 is 44. Of course there are losses using one battery to charge another, never disputed that. But not utilizing energy stored in my bank is wasteful- for the umpteenth time.[emoji48]. Bottom line is I'm just trying to make this stuff work. I need to charge my trolling motor at night. Guess what the converter works . I don't need lectures. One thing I know is taking one amp for 15 hours may take 15 amp hours out of a bank but taking 30 amps for 1/2 hour is not discharging 15 amp hours. That's what the chart shows. I can't find the graph I used to have but it showed how many amps you can take out at different rates and the higher rates were like 1/4 the amp hours of the lower rates.

[Bleemus]

Do you have a wind generator? Granted you won't get much from one at night in Miami but when those fronts roll through you may get some help at night.

[john61ct]

Originally Posted by sparrowhawk1

taking 30 amps for 1/2 hour is not discharging 15 amp hours.

Yes it is.

OK that is Peukert you're talking about.

The usual way to phrase what I believe you're grappling with is

The battery is rated at XaH, based on the 20-hour rate. So a 100aH batt should last 20hrs feeding 5A before hitting 10.5V.

The capacity of the battery is actually different when drawing at higher or lower rates.

Sorry for the "lectures", maybe others can try to help you without annoying you, I'll try to refrain for a while.

[sparrowhawk1]

Yes and it's been a pretty Breezy year. Small wind gen but my house demands are small too.

[sparrowhawk1]

This is similar to the Chart I was talking about. At 1 C you have to put twice what you took out plus account for inefficiencies.
And no worries. Internal combustion engines are only 20% efficient but that doesn't stop people from using them[emoji48]

[senormechanico]

Coming for help, then arguing with the help...

[john61ct]

Yep, what you are saying is wrong, and the chart has nothing to do with it.

[sparrowhawk1]

I'm being told I'm wrong but not why and being told I can't do things that I'm doing. Using the 100 amp-hour battery metaphor I can take 100 amps out for 1/2 hour. that's 50 amps correct? The battery is now dead needs to be charged back to 100% so that's 100 amps plus inefficiencies please tell me how I'm wrong. And yes let's please do this without negativity. is that too much to ask? I'm here to learn not to be insulted.

[john61ct]

To learn more effectively requires certain attitude adjustments. No insults nor "negativity" intended, maybe you could just toughen up a bit. Key is asking the right questions, and actively seeking learning resources out on the web, no one is going to spoon-feed you, lead you by the hand while you're kicking and screaming the whole way.

Originally Posted by sparrowhawk1

Using the 100 amp-hour battery metaphor I can take 100 amps out for 1/2 hour. that's 50 amps correct?

Yes.

> The battery is now dead needs to be charged back to 100% so that's 100 amps plus inefficiencies

Well not dead, with a quality bank designed for deep cycling, going lower will just shorten lifespan, just as only discharging to 20% will greatly increase it, 50% is just a rough guideline.

Same with getting to true 100% Full, that only needs to be a few times a week, and only for longevity. Many people don't, get by just fine replacing their batteries more frequently, it's a choice.

The key here is your acquiring the instruments and knowledge to **know** when your bank is at 100%, and to know how low your SoC is.

The former can be done precisely, the latter can only get to a reasonable guesstimate, depends on what you're willing to spend. Some try to do it just based on voltage. With FLA an hydrometer works. . .

I am imagining all the members laughing at me "he said he was going to stop!" just can't help myself 8-)