DC to DC converter

" Chris Wanker "

..... Phil

(balance of tirade snipped)

Even for a slow (> 10 hour) discharge, a lead acid battery's terminal voltage is about 1.75V/cell, or 10.5V for a 6-cell, '12V' battery.

Nominal cell voltages are just that -- nominal. Usually they're the unloaded voltage of the cell after it's discharged for a bit. If you want to extract all the useful energy in a battery, you have to take things much lower than the nominal.

More like they were designed to run off of nominal automotive voltages, with the expectation that you'd have the car running when you were in transmit.

I'd step up from below, either with a flyback converter or with an isolated output, connected between the battery's positive terminal and your load. Then the converter only needs to supply the difference between the battery output and 12V.

I haven't actually done it, so I dunno the right answer.

You may find that some of your gear works just fine down to the bitter end of your discharge curve, 11.7V rating or no.

"Tim Wescott" " Chris Wanker "

Standard lead-acid ( ie car ) batteries are NOT designed for deep discharge.

It destroys them very quickly.

An end point of 11.7 volts is a safe one and gets the most LIFE from such batteries.

Bet the Tx still operates fine at 10.7 volts too.

While destroying the damn battery.

...... Phil

Oh gee, Phil, it's so nice that you're willing to talk to all of us even though you're so much smarter than everyone else. You must be a social disaster if you can't find people as smart as you to hang with.

So, where _exactly_ does the OP mention that he's using car batteries in the original post? All I see is "lead acid", with no mention that they're not, indeed, deep discharge.

Deep Cycle or not,It's better not to cycle any lead acid battery below

11.7-15V sounds like the usual '12Volt/automotive use' voltage range.

If you have a good lead acid battery and you keep below C/10 amps, you should be above 11.7 for at least 50% of total capacity. If you are going to draw say C/5 amps then 11.7 will be reached in 30% of total capacity. Here are some curves that will give you what you need to know

Cheers

I think the "best solution" is to add a circuit that warns you to shut down - or automatically disconnects the load - when the voltage drops to ~ 11.7. The idea is to protect the battery when that voltage is reached, not to get more out of it. Discharging a lead acid battery too far damages it.

Ed

You can get a single cell SLA battery rated 5 A-H for about $10 (Mouser (846-0800-0004), but you would need to rig up a special way to charge it separately from the main battery, and it may not supply enough current. They have 25 A-H for $40.

You could also use two 4V and one 6V pack, but 8 A-H at 14 V will cost $65.

You can also buy a DC-DC converter with 2.5V 10 amp output (Mouser

Paul

Hey, Walmart has your meds for $4 now! (or maybe in AU it's Wallaby-Mart)

No more excuses...

But, of course, you will reply with "manly" vulgarity. I made you do it! Ha!

Paul

It'll be interesting to see the autopsy report on Phil's brain, when the time comes.

Bob

Maybe by then nanosurgical techniques will be advanced enough to detect both neurons.

Paul

It's not that he doesn't have enough neurons. He's a smart guy, but he can't control his temper. He seems to have a form of Newsgroup Tourette syndrome. I would imagine, however, that face-to-face he's a different animal. I'll bet he's a big old cuddly bear that likes being hugged.

I apologize for that last part.

Bob

I agree. He often gives very good advice, and he knows a lot about some things. But he is a "hothead", and it seems like he has spurts of being reasoanable and other times of just attacking anything and anybody, and he often singles out some people he just doesn't like.

Paul

Only if it's a bipolar bear.