Keeping a 12v battery charged above 80%

Yes, lead-acid batteries should be kept fully charged. The proper charger shouldn't be that expensive. Just keep it charged. Put it on a timer, if it'll be months without use; charge an hour or three a day.

I bought a Battery Tender Jr. for my motorcycle. A microprocessor-controlled marvel that usually did nothing at all, but sometimes it drained my battery below the level that it started at. Never once did it charge the battery. The "Lifetime" warranty required me to ship it back to Florida for repair, along with handling fees, diagnostic fees, return shipping fees, etc. that approached the retail cost of the charger. After a year, I smashed it with a sledgehammer and threw it in the trash.

I can certainly sympathize with that. I've had a few instances where I thought I might bring some faulty equipment into a mfger's office and drop it on the floor.

I own a cheap 4A Arlec charger with electronic control (electronic control comprises a pcb with a pair of SCRs, a transistor, a zener diode, and some resistors)

It charges to about 14V gradually reducing the charge rate. that would be stages 1 and 2 of the diagram here.

I use it for charging accidentally discharged car batteries, and for charging the sealed lead acid battery in childs electric trike. (it charges this smaller battery at 1A gradually tapering off)

It sounds like you'd want one that also does stage 3 "float charge" for your application. $150 does sound quite expensive (about twice what I would expect) but I don't know of a more competively priced off-the-shelf solution.

There must be dozens of good schematics for battery chargers on the web if you want to roll your own. Schedule? there is no schedule, but a fully charged battery will live longer. I had a Marine battery on my boat that lasted 12 years (through two boats). I put a Heathkit charger on it about once a month when I remembered to do it. Another on my sailboat that went 10 years on a solar panel and shunt regulated charger I built. It started the motor, kept the anchor light going, and cabin lights when I was on board I only checked the electrolyte level religiously and looked at the charger once in awhile to see that it was limiting current when I first got on board (every weekend)

Black and Decker used to have a selectable 1 and 2 amp charger. It turned off when the battery was fully charged. I've been using one of them for a number of years to keep the grandkids' battery-powered vehicles charged (beats using the "battery burner" that comes with the vehicles). The charger was about $30. I think they still have a similar one that only has the 2 amp output. That would be OK foryour application, but the small SLA batteries in the kiddie cars live longer if the charge is limited to 1 amp.

Ideally, a lead acid battery is never discharged below 50% - that's the standard used in designing the battery system for solar or other alternate energy. The less a lead acid battery is discharged, the longer it lasts - the electrochemical reactions (charge/discharge) are reversible, but there is a limit.

I repeat a conversation I heard recently between an ace mechanic and regular trailer camper, and a new trailer camper with a dead battery:

Go to a place that sells RV stuff and get a float charger. Figure on spending $20 to $25 bucks.

A "plain old" charger for lead-acid batteries will boil the electrolyte dry (well, it'll electrolyze it into hydrogen and oxygen, but it's the same difference to you). Go for the float charger.

How is charge measured? Voltage? Does 50% mean a 12v battery at 6v? That doesn't sound right.

Bob Engelhardt wrote in news:jr8d6d01ej8 @news3.newsguy.com:

That is because the talk was about the charge, not the voltage.

Below 12v you are trying to kill the battery. :)

--Winston

Integrate the current taken out of a fully charged battery (integral of current is charge). When that number reaches ~50% of the rated capacity, stop.

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I know. That's why I asked: "How is charge measured?"

More specifically:

Which says basically that charge is pretty hard to measure. I.e., there isn't a simple, accurate technique.

batteryuniversity calls that coulomb counting. Even that is subject to the battery actually having its rated capacity when it's fully charged.

Quite right. Thanks!

'Same is true for anything else we measure, for very demanding levels of accuracy. However, one can also take the article to say that measuring open-circuit terminal voltage (after allowing the battery to rest) yields accuracy that is quite acceptable for just about anything a hobbyist is likely to do.

Yes? No?

My nice torque wrench serves perfectly fine to be within +-5% of the *real* reading. I don't have a need to know my lug nut torque with much better accuracy than that. 'Same is true for my ammeters. I know that there is a burden voltage on top of calibration errors and the relative sloppiness of all physical things, but I accept their readings if I am comfortable that the reading that I see is likely to be 'close enough'. (Yes, rarely I *am* fooled by readings that are not nearly 'close enough').

Why take away these tools (and hundreds of others) as being 'too imprecise' if great accuracy is not required?

--Winston

Well, the unit of charge is the Coulomb, so that makes some sense. ;-)

If the battery is so bad that 50% isn't enough margin of error, it's done anyway. A battery is considered "bad" if its capacity is 80% of that of a new battery.

Read the wiki:

When a lead battery starts gassing it is full.

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Yep. And when I needed the warranty (5 years I think it was, not lifetime), I found it was laughable.

People also like to push their choices, and rally others to follow. Everyone likes to be an authority. I see that so often on hokey Newegg.com reviews.

Wrecker makes a good point. I see tons of reviews where the buyer just unwrapped the present and talks about how well it works. OTOH, I generally look at the negative reviews. I want to see what problems were encountered and how they were fixed. If the seller takes care of the problem, that's a positive in my book.