Battery Charging

[sjohns4]

1) Can a basic unregulated DC power supply be used for battery charging (lead-acid type)?
2) is there a rule of thumb or formula to size the current output of the power supply to maintain a battery at or near full charge without over-charging?

 

[MikeHalloran]

1) Yes, if you only need the battery charged once and you have no alternative anyway.
Decades ago, the answer was simply 'yes', and you could buy a decent charger in a drugstore for a couple of bucks, and it would work okay because all lead-acid batteries were pretty much the same. Now, there are a fair number of different types of lead-acid batteries, and they don't all behave the same.

In any case:
DO TAKE PRECAUTIONS TO PREVENT HYDROGEN EXPLOSIONS,
AND TO MITIGATE THEIR EFFECTS >>WHEN<< THEY OCCUR.

2) In days of yore, a trickle charger rated to push in a full charge in a couple tens of hours would be about right. ... but you still had to check for gassing, temperature excursions, and electrolyte loss.

Nowadays, you can buy specialized chargers, each optimized for different service, and for specific types of lead-acid batteries.

I.e., a power supply sized to a rule of thumb is very unlikely to give the best possible results, or even marginally satisfactory results. Batteries never were well-behaved loads, and they're getting more complex and more picky about how they are treated, and what you do with them. Since you haven't revealed squat about what sort of battery you have and what you are doing with it, you probably won't get step by step help here, except to read the mountain of information available from manufacturers of batteries and chargers.




Mike Halloran
Pembroke Pines, FL, USA

 

[DRWeig]

Mike gives good advice, pay attention to it.

My comment is that you seem to be re-inventing the wheel. This problem has been solved, find a few of the known-to-work circuits and work it into your design.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[sjohns4]

It's somewhat of a two-fold question.

First, an RTU manufacturer was telling me if we need the ability to call out an alarm longer than their standard ~16 hour battery backup system then all we need to do is get a bigger battery, and they often use/see marine deep cycle batteries in remote boxes rather than the small battery that fits in the enclosure. Smelled kind of fishy. In this case I'd think the power supply/charger would be a bit undersized. The RTU is 12VDC powered, so the battery is wired in parallel with the power supply.

The second, and way more general reason: I have several batteries that sit idle all winter, 1-marine deep cycle and 3-motorcycle sized. Currently I'll rotate a small trickle charger between them while not in use. I've got a handful of 12VDC low output power supplies from various deceased electronics laying around & wondered how much damage I'd do by wiring them up and leaving them be for long periods of time. Maybe I should just quit being so cheap though!

 

[IRstuff]

"The RTU is 12VDC powered, so the battery is wired in parallel with the power supply."

That may simply be your impression. While battery charging is non-trivial, it does not require much of a footprint; this is why you can find battery chargers that have essentially the same volumetrics as wall warts.

I guess I don't necessarily see the disconnect with battery size in your first sentence. Assuming a certain capacity for the standard battery in this application, if you want longer durations, then you need a larger battery. However, that doesn't mean that the charger is undersized; it's only undersized if consumption of capacity is chronic and frequent. Otherwise, the fact that it might take twice as long to recharge the battery may be irrelevant, because it's covered up by the time required to clear the condition that caused the alarm in the first place, and id the repair is done correctly, the long alarm condition ought not re-occur that soon, particularly since one tends to be more vigilant afterwards.

Unless the larger battery has massively larger self-discharge, any trickle charge can be trivially accommodated, since the worst case is that the charger actually charges instead of trickle charging.

TTFN
faq731-376

 

[sjohns4]

It is not my impression. It is how it is wired. The conductors from the power supply terminate on the same RTU terminals as the battery leads.

It makes sence that a larger battery would take longer to charge than a smaller battery with the same charger. In this case that isnt an issue since the battery is only there for a utility power failure. During those times the RTU will call out via radio if the water level get's too high so the operators know to bring out the generator or diesel pump. It does have me curious now however. If what if the OEM battery were replaced with a smaller one, could the current output from the power supply designed for a larger battery damage the smaller battery?

 

[IRstuff]

No, because all these things operate on VOLTAGE differences, not current. There are current-mode devices, but none of them would be applicable in this application. Automotive and similar acid-chemistry batteries are much more forgiving with respect to chargers, and are much different than NiCd, NiMH, or LiPo, batteries, which do not tolerate much abuse or charging outside of their standard parameters.

TTFN
faq731-376

 

[sjohns4]

OK, so with a basic power supply the current decreases as the battery voltage increases & becomes closer to the power supply's voltage. This makes sence: I recall the old "dummy" type car battery chargers which arent anything more than a transformer and rectifier would show on the meter close to the charger's rated amperage on a dead battery but it would slowly climb down and once charged would be down around 2 amps or so (on my 10-amp charger).

Once the battery's voltage reaches that of the power supply's output, will the current drop to or near 0?

 

[IRstuff]

Yes, again, most power supplies are voltage difference driven and the basic Ohm's Law equation dV/R = dI would apply. dV = 0 would result in dI = 0, which is basic physics.

TTFN
faq731-376

 

[DRWeig]

all we need to do is get a bigger battery, and they often use/see marine deep cycle batteries in remote boxes rather than the small battery that fits in the enclosure. Smelled kind of fishy.

In my experience, adding external large batteries is not fishy at all, especially when the manufacturer of the backup system says it's OK. We have done that many, many times with hospital building automation systems and fire alarms. Even way back in the olden days, a shelf full of Sears & Roebuck Die-Hard Marine Deep-Cycle batteries provided whatever we needed.

Plus, most data-center class UPS systems come with lugs for external banks, and they sell them as optional adders.

In your situation, I would just hound the manufacturer for some sort of official data that says it's OK with their charging circuit, and go with it. They'll probably accommodate you.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[sjohns4]

Reflecting a bit, I probably was making this a bit more complicated than it really needed to be. After all, most deep cycle batteries in boat/RV applications live a hard life of varying discharge and charging and provide a few seasons of good reliable service. Thanks for the input though, now I know for sure!

Mike

 

[waross]

A couple of comments;
AC powered battery chargers charge with rectified AC, not smooth DC. If you have a power supply that outputs smooth DC set it for about 13.8 Volts and expect the same reliability that you get in your car where the battery is charged by an alternator that puts out 13.8 Volt DC, not rectified AC.
Size: Some battery chargers and some alternators limit the voltage, not the current and may be overloaded and damaged trying to charge a discharged battery that is much larger than they are designed to charge.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mikekilroy]

Read this if you don't want to dance on the head of a pin, never fully charge your battery, or cook your battery.

http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery

Then every now and then (few weeks) as you walk past the battery bank check the batteries temperatures with an IR gun to make sure they are cool not warm.

 

[VEBill]

Allow me to clarify this one point for others that may read this thread in the future. This does not apply to the circumstances of this alarm panel as subsequently clarified.

1) Can a basic unregulated DC power supply be used for battery charging (lead-acid type)?

Maybe not.

If you have a lead-acid battery and it's uncharged, then it may present what is essentially a dead short to any applied voltage source. A battery charger will limit ultimate max current to protect itself; a "basic unregulated DC power supply" might not. A simple DC power supply might be overloaded, would potentially overheat and fail. A better power supply would self protect.

Your alarm panel would have a battery charger in it, so this caution dies not apply.

 

[VEBill]

PS: one could insert a suitable series resistance into the charging circuit to limit the max current into a flat battery. In a pinch, a suitable light bulb wired in series is often the simplest solution.

 

[waross]

In a pinch I have charged batteries with a light bulb and a diode.
In Central America many fishing boats and lobster boats have a 32 Volt electrical and starting system. The electronics are generally 12 Volts. The electronics are generally powered by a 12 Volt 8D battery. They use a 24 Volt light bulb between the 12 Volt positive and the 32 Volt positive. This keeps the 12 Volt battery charged and there is always an emergency light on in the engine room.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mikekilroy]

hmmm... I have never seen a dead lead acid cell look like a short circuit, but perhaps i have never been in the right place at the right time.... my lead acid cells all look like open circuits when dead, and as the voltage quickly rises, so does the current in even slower process that has never presented anything near high current. If such a dead lead acid battery can and does revive, the current eventually rises to amount required to charge it but only after the cell voltages gets over 2volts each anyway (if over 2.3-2.4v per cell then full charging can happen). I have a habit of leaving my 2 meter on in the car in the garage siting for days on end, and have replaced the batteries over the years with new ones and made them dead too. so from worn out to brand new but dead, no high current in my experience yet.....

as an aside, i have revived lead acid batteries by the diode in series with light bulb and let the 60x per sec hits smack at the sulfided battery for 1-2-3 weeks at a time (it starts at 0.x ma) and often the old no good battery comes back (as long as it is not yet crystallized).

 

[VEBill]

"[Interconnecting 32v and 12v systems] ...They use a 24 Volt light bulb between the 12 Volt positive and the 32 Volt positive. ..."

And what could possibly go wrong with that? Given conditions that could easily occur, can you see that the 12v system might rise to 32v? Is there a voltage limiting shunt across the 12v system to prevent this obvious outcome?

"...never presented anything near high current."

Given (for example) a typical lead acid car battery, what is this self limiting charging current, given a voltage source?

For a real world example, if you apply jumper cables between two cars and allow one to recharge the dead battery in the other (all loads off, just charging), roughly how many amps do you think is flowing in the cable? 0.1? 1? 10? How long does it take?

Can you see that there might be certain circumstances where a "basic unregulated power supply" could be overtaxed by connecting it straight to a lead acid battery?

 

[waross]

It's pretty hard to get an 12 Volt 8D battery much above 14 Volts. The system works quite well. I can't say as much for the manual field regulation that they use in place of the factory voltage regulators on the engine alternators.
Back in the day of generators to charge the batteries any the vibrating point voltage regulators, someone came up with a manual adjustable field controller. When alternators took over from generators the practice persisted and most of the electronic voltage regulator/field controllers were replaced with the manual units. They burn out a lot of alternators. On one small island a repairman makes a good living rewinding alternators that have been burned up by the manual controllers. If the captain advances the throttles a few hundred RPM and forgets to have the engineer trim the manual voltage control the alternator often goes up in smoke.
By comparison, the light bulb chargers are a marvel of dependability.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[OperaHouse]

Light bulb,...Heck, I once charged a battery with a toaster.