24 volt welder- charging batteries

[coachgeo]

Unfortunately I gotta work on the cheap. I got some shop space now and starting to set up my welder. It is a portable 24v that you hook to two auto batteries. It is a Ready Welder Prototype. For my needs this should work.

. Anyway........ keeping two junk yard batteries charged.... what do you folk suggest?
. Do you think I need to use two equal type batteries, (age, manufacturer, CCA's, etc.) .
. Can I charge two batteries from one charger? Time to charge is not really an issue.

[Sandy]

Ideally you'd have a matched set, but I don't really see where that's necessary. Same series would be nice too, but then again you deal with what you have. I'd try for the same series and about the same condition if you're stuck with used ones.

When it comes to charging two with one charger, I'm not sure anymore. So much of that is going to depend on your charger size and technology. WIth auto sensing and such, I don't know that your charger could ever really regulate correctly and then fall back into the proper "trickle" or "maintainer" mode unless they were identical in characteristics.

Give it a shot for a few minutes, then stretch it out. I think your charger is going to try and average the characteristics.

[awright]

While acknowledging your motivation to minimize costs by using two unmatched used batteries, you should know that using batteries that are not very well matched in a series, high-drain arrangement is risky. Lead-acid batteries are easily damaged by discharging below a specific voltage that occurs well before full discharge. Having one strong battery in series with a weak one risks over-discharging the weak one and damaging it further, possibley terminally. However, the batteries should be OK if you install fairly inexpensive discharge warning devices on EACH battery. Such warning devices

[awright]

While acknowledging your motivation to minimize costs by using two unmatched, used batteries, you should know that using batteries that are not very well matched in a series, high-drain arrangement is risky. Lead-acid batteries are easily damaged by discharging below a specific voltage that occurs well before full discharge and it will take extra complexity to monitor voltage on each battery in a series connection. Having one strong battery in series with a weak one risks over-discharging the weak one and damaging it further, possibley terminally. However, the batteries should be OK if you install fairly inexpensive discharge audible warning devices on EACH battery. I have seen such warning devices sold at marine supply stores.

Charging two unmatched batteries from a single charger is even more risky re: battery life and will limit your ability to optimize the charging cycle for each battery. You would benefit by getting the booklets on battery care and charging from the manufacturers like Yuasa. If you google, "charging lead-acid batteries," you get more than two million hits. There is probably more info on the first page of hits than you will ever need. The main information you need is the critical nature of charging current and voltage and the phases of the charging cycle. That is, (as a broad generality) you can fast charge up to a certain voltage because all or most of the charging energy is going into the chemical reaction at the plates. Above a specific voltage (that is dependent upon battery temperature and battery chemistry) more of the energy starts going into heat and electrolysis of the water and current must be limited to avoid damage to the plates and loss of water. As full charge is approached, current must be tapered off further, but it is beneficial to continue with a "finishing charge" to achieve full capacity from the battery. Charging must be terminated at a specific voltage to avoid permanent damage to the battery. You can see that you will be completely unable to even approach such optimized charging with two unmatched batteries either in series or parallel.

The one configuration you could use to charge two batteries from a single charger is to use a power supply with a hard float voltage limit. You can charge multiple batteries of similar chemistry this way because the permissible float voltage is lower than the damage voltage for continuous charging. The downside is that you get much less than full capacity out of the battery using this technique because you never get the "finishing charge," and charging is slow. I believe that this kind of float charging can be done without a current limit because the charging is self-limiting at each battery as the battery approaches the fixed float voltage, but this should be checked with the manufacturer's literature.

Another downside is that you can't leave the batteries in parallel on float charging during use because your application demands series connection of the batteries. That means repeated reconnection of batteries and batch charging.

All-in-all, I think you are asking for trouble by trying to charge multiple, unmatched batteries with one charger. Can you afford separate chargers for each battery in the series stack of batteries, along with discharge limit warning devices for each battery?

Good luck. I have a Ready-Welder myself that I have yet to power up.

awright

[coachgeo]

... Can you afford separate chargers for each battery in the series stack of batteries, along with discharge limit warning devices for each battery?Possibly. The audible warning device is of no value to me. The shop I'm renting is miles away from my house and work and I can't take things to work to charge as I am teaching.

[awright]

The audible device I was referring to is to warn you that you are about to DISCHARGE one of the batteries below the damage threshold, not that you are charging too much. It would presumably only sound while you were using the welder and were, therefore, present within hearing range of the audible alarm at the battery pack. Its purpose would be to warn the operator to stop welding until the batteries could be recharged.

Charging by whatever means you select should be fully automatic, charging each battery separately with suitable battery protection built in to the controller.

awright

[coachgeo]

ahhhhhh audible device is for drainage protection. Makes since. Where do I find such beast, and what is their common name so when I ask for them the counter person/order desk knows what the heck Im talking about?

[awright]

The place I've seen such devices is in marine supply stores like West Marine. I don't do boats (I get seasick), but I do like to check them out occasionally for interesting stuff. I presume that they are desirable on boats where you could be running accessories off batteries while the engine is not running.

I don't know a specific product name, but "audible battery monitor," ought to be descriptive enough. I don't know where you are or if you have marine supply stores nearby, but West Marine is nationwide, I think, and they have a web site with on-line catalog.

BTW, if you can't find one, we could probably come up with a circuit for one if you wanted to tackle circuit assembly.

awright