Battery charging question, Hopefully not boring

[larrybl]

Hello All, I figured this might be a good place to enlist some extra grey-matter for a project I am working on. I want to add LED backup-lighting to my shop. Sort of falls in line with Dave’s recent battery charging videos.

I have a 18X24’ shop and I have acquired some free 10X3 LED lamps, (They are from street lamps that were knocked over). They are best operated from an 18 VDC, 1 amp supply.
I have a wall-wart that is supposed to supply 18VDC at 1A, but I only measure 16 VDC. I can run both lamps from the wall-wart and the light output is acceptable, but the wart voltage is around 14VDC at 3/4A. It gets warm but not hot.

I purchased two sealed led acid batteries, 6V @ 2.9ah and 12V @ 2.9ah. I hooked these in series for 18V and powered the lights for almost 2 hours (just what I want). Now how to charge the batteries back up.

The wall-wart only has 16VDC (although rated for 18VDC) so I added a 820 uF cap that brought the voltage up to 24V. I placed a small 12V lamp in series to limit the charging current. So far it seems to work. I have been charging the batteries at 18V for almost an hour and the voltage is staying steady at 18.5 VDC and the limiting light bulb is getting dimmer.

I want to finalize the charging design, and make the lights come on automatically with a power outage. Suggestions are appreciated. I mostly deal with transformers, diodes, caps and resistors, Please no exotic recommendations.

Some background, I used to repair Valve / Tube TV’s, built my own Valve / Tube amplifier. I work on 12VDC automotive / Tractor systems, maintain a Public Safety Radio System (my day job) and I built and wired my shop.

Really enjoy this forum, and Dave’s videos.

Hopefully the Pictures will show what I am trying to do.

[Gyro]

I wish they used LEDs in our street lamps - it might be worth knocking a few over then. 

Sorry to disappoint you, but your wall-wart isn't up to the job. To fully charge 18V worth of Lead Acid you need to get them to roughly 20.7V. Your lamp is dimming because you are getting up to partial charge on the batteries but they have a long way to go still.

Edit: If you want a really simple way of automatically switching the lights then you probably can't beat a relay. It needs to be upstream of a diode in the charger circuit so that it switches with the mains and use the N/C contacts.

[Simon]

I purchased two sealed led acid batteries, 6V @ 2.9ah and 12V @ 2.9ah. I hooked these in series for 18V and powered the lights for almost 2 hours (just what I want). Now how to charge the batteries back up.

Bad idea, lead acid batteries in series or parallel in only ok if they are identical batteries and are put in series from new. Check the voltage across each battery to make sure all is well. I suppose a zener could be used as safety net but thats not good either,

[oldway]

Not bad idea at all.

There is no problem to put batteries of different voltages in series.
What is indispensable is that these batteries are of the same type, the same capacity in ah, the same initial state of charge and incidentally of the same manufacturer.

The 2 batteries are of the same ah (2.9 ah), that's allright if the 2 batteries where initialy fully charged.

[Simon]

Fine, they are the same capacity. So long as that is understood. I tried a 45Ah and 60Ah batteries at work - I had to monitor them constantly.

[larrybl]

The batteries were bought "new" same manufacture "Duracell" and same 2.9ah on each battery. I checked the charge on each this morning, 6.1 and 12.2. The unloaded wall wart (with the additional cap) shows 24V. I'll remove the ballast lamp and charge the batteries for another hour to see if I can get the voltage up a little more.
Yes, I was thinking of a latched 120V relay for the automatic switching. thanks for all the comments. I figured the battery charge circuit would be the most challenging. I do have a 28VAC center taped 3A transformer, bridge rect, and LM317t that I could probably do something with if the wall pack isn't up to the task.   

[Simon]

Was your 18V transformer just a transformer and no smoothing capacitor in the first place ?, most car battery chargers of old were just a transformer and recitfier, they didn't really bother with a ballast. Full charge will be obtained with 14 and 7V so really uou need a 21V transformer, or rather a 22.5V as 1.5V is lost in the rectifier.

[oldway]

Fine, they are the same capacity. So long as that is understood. I tried a 45Ah and 60Ah batteries at work - I had to monitor them constantly.

They are not the same capacity, to connect them in serie is a bad practice.
You will kill the 45 ah battery.
When the 45 ah is fully discharged, is you go further, you will invert polarities on the 45 ah battery and dammage it.
If you try to recharge the two batteries in series, you will heavily overcharge the 45 ah battery.

[Simon]

I know, hence i monitored them and used them for short periods only. I was told by a narrow boat owner about how he just added a second 12V battery to his boat (in parallel I think) the result was that at least one blew up!

[larrybl]

"Was your 18V transformer just a transformer and no smoothing capacitor in the first place ?, most car battery chargers of old were just a transformer and recitfier, they didn't really bother with a ballast. Full charge will be obtained with 14 and 7V so really uou need a 21V transformer, or rather a 22.5V as 1.5V is lost in the rectifier."

The wall pack has an internal transformer bridge and 25v 3300uf filter (I know because I took apart a 12V 1.5A pack). Stupid thing had an internal glass fuse, and the unit was sealed requiring abuse to open it up. I replaced the fuse with a more common type and taped the unit back together. The 18v pack is similar in weight and manufacture, so I assume it has similar bridge and filtering. 

It looks like the 18v series charge has leveled out to 19.5v, I'll dig around in my parts to see if I have a transformer that may be better for the task.   

[Simon]

The wall pack has an internal transformer bridge and 25v 3300uf filter (I know because I took apart a 12V 1.5A pack). Stupid thing had an internal glass fuse, and the unit was sealed requiring abuse to open it up. I replaced the fuse with a more common type and taped the unit back together. The 18v pack is similar in weight and manufacture, so I assume it has similar bridge and filtering. 

Does not follow at all. Some wall adapters are just transformers, some just a transformer and rectifier, some a transformer, rectifier and smoothing capacitor, others even are SMPS, they will not tell you any of this on the markings, only way to find out is take it apart or check the output with an osciloscope.

If there is no smothing capacitor you will get the RMS voltage of the transformer minue the diode bridge (with just a multimeter the bridge drop will probably be nill) so the peak voltage of the transformer is what you measure x 1.414

if there is a smoothing capacitor the voltage you read when unloaded is the peak and the RMS voltage is what you read / 1.414

[larrybl]

OK, ditched the 18v wall pack and pulled out my old test power supply, I added a 820uF cap for filtering. unloaded I am getting 22.2vdc, attached the series battery and the voltage is climbing. Should I add the ballast lamp?

[Simon]

Yes put the ballast lamp on.

[larrybl]

OK, ballast lamp is not lit, and charge voltage is at 20.6 (full charge)  . It is a rainy dreary day here, and I have a hard drive full of Dr. Who seasons, so I now plan to run the LED's again and document the Voltage / current draw every 15 min to see the actual discharge rate. Please stand by.

[Simon]

OK, ballast lamp is not lit, and charge voltage is at 20.6 (full charge)  . It is a rainy dreary day here, and I have a hard drive full of Dr. Who seasons, so I now plan to run the LED's again and document the Voltage / current draw every 15 min to see the actual discharge rate. Please stand by.

the ballast lamp will hardly light unless under a fast charge depending on the lamp of course, it's more of a safety measure than charge control. The voltage on your battery is more telling. Cars charge to 13.8V usually, a full charge is deemed to be obtained at 14.4V

[larrybl]

"the ballast lamp will hardly light unless under a fast charge depending on the lamp of course, it's more of a safety measure than charge control. The voltage on your battery is more telling. Cars charge to 13.8V usually, a full charge is deemed to be obtained at 14.4V"
I am well versed in 12V battery charging for Lawn tractors, wasn't sure the sealed batteries acted the same. I added some 28V LEDS to the Red Garden tractor, (ask me how I did that). Any how, the discharge test is on as of 10:30 my time. 18V - 1.7A draw.

[Simon]

Yes all lead acid batteries act the same, it's just that they have different charge and discharge carachteristics but it's all the same.

[Ian.M]

+ you cant top up a SLA with distilled water so every time you over-charge it, its another nail in its coffin.

[larrybl]

" + you cant top up a SLA with distilled water so every time you over-charge it, its another nail in its coffin."

Yes I agree, this is why I am trying to be careful here.

While we are waiting, there was another theory being offered. If I power them directly from the GT battery with less than a foot of wire they will light up off 12V, I have about 30' #24 AWG of wire on each (what I had available at the time) and it takes 18V now, but the amperage is the same for both. It might be possible if I run #10 or #12 AWG wire they might work off the GT's 12V battery. (I have a power port available that I run a 400W inverter from). Guess I need to dig through my wire box and give this a try. 30 Minuets and they are still bright, 17v - 1A.

[Simon]

According to the attached you have 1.33 ohms of cable resistance, at 1A that is 1.33V, not much, but a bit of a waste. go for 20AWG

[larrybl]

Ok 1:15 hour test results.
10:30 18v - 1.6A
10:45 17.5v - 1.3A
11:00 17v - 1A
11:15 16.8v - .55A
11:30 16v - .25A
11:45 15.5v - .1A

At 11:45 the lamps would still provide usable light in a dark shop, but I didn't want to draw the batteries down too far. I have a 10 day return policy on these.
I'll now take a lamp down and upgrade the wire to see if I can get it to run off the GT battery.

[Simon]

You don't really want to discharge past 6 or 12V if you want to batteries to last long.

[larrybl]

I tried thicker wire, direct to the GT battery. no-go so anything I do will be at 18v. I don't think the wire thickness affects them much. Back to the battery charger design.

[Simon]

No wire thickness won't make much difference but 1.3V on 18V is a 7% loss, so best increase to at least 20AWG. Your fully chrged 12V batter should sit at 12.6V when at rest and under no load, at 12V you can consider it discharged. going lower often will just exaust battery life span too quickly.

[larrybl]

Been on the make-shift charger for an hour, 12.1 and 6.0 respectively. I am using the center tap off the transformer (14vac ~2A) feeding a bridge rectifier and a couple filter caps to give a no-load of 22.2vdc.
Should I move to the full transformer windings (28vac ~2A) and incorporate the LM317t that I have? I know the 317 is rated at 1A with a heat sink, so that it will only charge the batteries, and not power the LEDS.

[Simon]

once you try to charge the batteries you will fall towards the 14V again as the capacitor will not hold the full voltage while under load. You could use the regulator to supply 20.7V (ok 21V) that way they won't overcharge. You might want to check the charge current so you don't overload the regulator and transformer or use a ballast bulb again.

[Ian.M]

Probably simplest to use an adjustable CC/CV buck/boost module off EBAY.
e.g. http://www.ebay.com/itm/DC-DC-Adjustable-Automatic-Boost-Buck-Converter-2A-10W-CC-CV-Voltage-Regulator-/171528807479 or similar.

It connects directly between your power supply and battery, and you simply set the float voltage into a light resistive load and the max charging current with its output shorted through an ammeter before hooking it up.  As with most other Chinese ebay crap, the power and current ratings are extremely optimistic, so for that module, I wouldn't set the current greater than 300mA, a bit over C/10 for your batteries.

The problem with a LM317 based charger is heat dissipation.  Also you may have problems at switch-on if the batteries aren't connected, or if the output ever gets shorted as the peak rectifier output from a 28V transformer would be right on the bleeding edge of the LM317's abs max Vin to Vout rating.

[larrybl]

OK the batteries are at 12.4 and 6.2 close to a full charge. With the thought of wanting to run these off the 12v GT battery thus eliminating the 18V battery and charger issue, (and re-cooping the $40.00 in battery cost), is there a simple voltage doubler circuit I could build to power these off the GT's power port?   

[Simon]

You need a boost type switchmode converter. Not the sort of thing you just cobble together. You need to buy an off the shelf item or get a control chip and follow it's datasheets recomendations for the rest of the parts.

You will find an off the shelf solution in the form of an in car laptop power supply, these take in 12v and usually output 15-24V depending on a selector switch so right up your street.

[larrybl]

Yes! "You will find an off the shelf solution in the form of an in car laptop power supply, these take in 12v and usually output 15-24V depending on a selector switch so right up your street."
Thank you, I will return the batteries tomorrow, I did a search for "Car Laptop Power Supplies" and found a wide varity ranging from $5.00 Hong Cong, to $35.00. I think the $19.99 off of Amazon will work nicely.

Output: 19V 4.74A 90W (18.5V, 19.5V / 65W, 45W compatible) / Power laptop and charge battery from 12V DC outlets (car / air / boat) / Check Connector Photo for Compatibility (2-nd Photo)
• Pwr+ DC adapters manufactured with the highest quality materials and include laptop safeguard features against incorrect voltage, short circuit, internal overheating. 100% compatibility with the original.
• Products trademarked Pwr+® are marketed and sold exclusively by Pwr+. We focus on providing quality power products and excellent customer service ... and we also ship your order the same or next day (Amazon order processing time varries) !
• Warranty: 30 Days Money Back Guarantee / 24 Months - Free Exchange

[Simon]

Ultimately you get what you pay for and if the seller is in the same legal juridstiction you have "some" recourse.