SLA Charger Question

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Transistorized

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Ok. Most of you guys know that my DJ-Tech Boombox's internal circuitry isn't charging my battery fully (12.4V). This is resulting in less play time and the battery status indicator on the box always being one light down from the start. I am still able to get a few hours of play time using the internal charger but if it was charging properly I could get 10hrs +

What I wish to do is bypass the internal charging circuitry by tapping directly onto the battery terminals inside the box to a female barrel plug adaptor mounted externally on the box. Then using a male barrel plug, supply charging current directly to the battery. I'm smart enough to get polarity correct and maybe use a diode in the circuit (for reverse polarity safety) and a 5A inline fuse just in case of a short.

I see all kinds of SLA battery chargers on eBay. They are advertised as 12V SLA battery chargers. I want to stick with a charging circuit of around 600mA but no more than 1300mA. My problem with these chargers is the output voltage. They all say 12V. My battery already gets up to 12.4V (not fully charged) and shows partially discharged on my meter of my box. I need at least 12.6 to 12.7 resting volt before the battery status indicator shows full.

My questions are:
1: Don't the chargers need to be at least 14V to get the battery topped off with a resting voltage of 12.7 to 13.29? Otherwise my meter will just show partially discharged as it is looking at voltage.
2: Do I need to continuously monitor the charging process manually doing it this way or will the output of the charger and the battery equalize when the battery meets the chargers output voltage? I don't want to overcharge.
3: Is this a bad idea?
 

Transistorized

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Update:

I think I may have come up with a free way to top off my battery safely. I have a Stanley Jump box used to jump dead car batteries. It has a slow charge wall adaptor that charges the SLA battery in it. It's 12V with 200mA output. Im alittle confused because when I tested the voltage without a load from the wall adaptor it reads almost 17.71V. Im guessing this has to do with the load of charging the battery evening things out because it does say Output 12V 200mA. I already have a female barrel plug with terminals so I found some wire and tested my contraption on the Power King battery that was in the DJT box.

Pictures Below. What do you guys think about this? 200mA is a slow charge but should work nicely. I don't think this would overcharge the battery. I'll keep amd eye on the voltage and see what happens.

20161210_131601.jpg

20161210_131536.jpg
 

jimmyjimmy19702010

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The '12volt' battery chargers you're talking about are referred to as '12 volt' as they are designed to charge a 12 volt battery.

Any voltage under 12.90 volts will not do any charging. Whilst any voltage above 12.90 volts will charge the battery, most chargers will provide around 14.10 volts to charge the battery in a reasonable amount of time.

My charger: https://www.jaycar.com.au/2-6-12v-lead-acid-battery-charger/p/MB3518

This is a 600 mA 12v charger. Remember, the bigger the battery, the bigger the power amperage it can safely handle whilst ensuring a long life and thus a maximum number of cycles.

Your battery is smaller so I'd advise not to go over that charge rate. A lot of the cheapy chargers charge at 1.2 amps - sure, it will work but the battery won't like it. An 'automatic' charger that constantly monitors the battery charge status and automatically reduces the charge current is best.

Remember also, many of the 12v wall warts you may have around the house aren't strictly chargers. Sure they provide power but they don't contain the additional battery charge management circuitry required to properly charge and then maintain the battery voltage.

More info: http://www.powerstream.com/SLA.htm

James.... :-)
 

Transistorized

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Hi James. Something just seemed off so I kept looking (we cleaned our house up) and I found the correct charger for my Jump box. Even though it shows 12VDC it outputs 14.4VDC at the terminal at 500 MA pictured. Do you think this will be OK? I've checked inside and the jumpbox has no circuitry to control charge. It goes straight to the battery terminals inside and I've left it plugged in for weeks without overcharging that SLA. Let me know if I am missing anything because I plan to give this a go. Wish me luck ;-)

View attachment 31486
 

jimmyjimmy19702010

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Transistorized said:
Hi James. Something just seemed off so I kept looking (we cleaned our house up) and I found the correct charger for my Jump box. Even though it shows 12VDC it outputs 14.4VDC at the terminal at 500 MA pictured. Do you think this will be OK? I've checked inside and the jumpbox has no circuitry to control charge. It goes straight to the battery terminals inside and I've left it plugged in for weeks without overcharging that SLA. Let me know if I am missing anything because I plan to give this a go. Wish me luck ;-)
20161210_152119.jpg
By the sounds of it, that should work fine. The 500 mH charge rate certainly won't hurt your battery. I wouldn't leave it connected after charging though. Just let it charge for 12-24 hrs and then disconnect the charger.

Have fun,

James.... :-)
 

Transistorized

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This was a success. I went ahead and purchased a smart charger so I don't have to baby this thing while charging but here are the modifications to my box. I didn't want to drill holes in my new "defective" box so I found a location that I will later glue the female adaptor.

Below are pictures of all I did. I wired the charging system to where it uses the original 5A fuse. This way if something gets shorted on my end it will pop this fuse. At this time there is no reverse polarity protection but seeing as this will always be my box I don't have to worry about ill informed customers who might not know. If I ever sell this box I'll place a diode capable of handling 5A.


Here I have the connector off the board, the wires removed from it. I lightly soldered my leads to the terminal ends.
1.jpg

Now I have the plug back on the end of the battery cable with my tap installed
2.jpg

Everything secured and zip-tied
3.jpg

Location of charging port. (Did NOT want to drill any holes and this is relatively hidden. No modifications to case)
4.jpg

In addition to being able to properly charge the internal battery, I can monitor the voltage on the fly.
5.jpg

I have a smart charger coming so I can just plug it in and walk away. When I go to grab my box from now on, it'll be ready to rock for hours and I can always monitor the actual voltage so I know where my battery state actually is :-)

I am going to move the charging port further back in the hole so it's less noticeable later.
 

jimmyjimmy19702010

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Where does the original charger plug in? Is there any way you can position your new plug in that area. A partially blocked bass port may affect the bass reflex port tuning.

Your new wiring to the battery looks very 'factory'. Nicely done. :-)
 

Transistorized

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jimmyjimmy19702010 said:
Where does the original charger plug in? Is there any way you can position your new plug in that area. A partially blocked bass port may affect the bass reflex port tuning.

Your new wiring to the battery looks very 'factory'. Nicely done. :-)
Thanks :-) The original is in the back of the box. Most all of the other areas are now covered by the upgraded battery or other circuitry. I thought about possibly drilling a hole through the side or back and going to RadioShack and get a female plug with a collar that screws in. The wood sides and back have a vinyl wrap that I felt would rip or tear with drilling. The plastic or metal on the front might be the way to go for a more permanent fixture. At this point nothing is permanent yet. I did this because I wasn't sure how this would turn out and I wanted it to be something that could be undone without scarring the box if needed.

Even though my new box was crippled with this plague, my goal is for it to look like nothing happened and to hide the charging port so as to go unnoticed. So far I haven't noticed a degradation of sound. Actually the unit sounds better. My guess is because its getting a properly charged battery voltage now :-) Plus if I want I can still use the factory internal charger in a pinch.

My hopes of having this box as a long lasting portable are (for the most part) complete. With being able to monitor the voltage I can be sure she's tip top when I take her out for jamming :-)
 

Superduper

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The wall wart type units you showed aren't battery chargers. They are power supplies. Technically speaking, any power supply that outputs more than the battery voltage of the battery you are charging will serve to charge it to some extent. However, unless you strictly set the power supply onto a timer, you can't really control whether the battery is undercharged or overcharged.

The reason why you are getting readings of anything from 14 to 17 (even though power supply display rating of 12v at some current) is that these are UNREGULATED power supplies. The ratings, supposedly, are accurate when it is outputting current at the prescribed current. In other words, 12v at 500mA would mean that it's output voltage will be 12v when outputting 500mA. Since your SLA battery requires 12.6v approximately at rest, it is impossible for that charger to charge at a 500mA rate since at that current draw, the voltage will drop below the level need to charge the battery, at least not unless it is severely discharged. I'm not saying it won't charge. It will but it won't charge at 500mA unless your battery is severely discharged. At some other charge level, the charger will charge slower, perhaps considerably slower than .5A. Anyhow, since the spec is only a snapshot spec based on a particular current draw, the actual voltage will vary depending upon the battery's current charge level. It is prudent to have a voltmeter connected during charge sessions so you'll know what's going on. When fully charged, I would not be surprised to see voltages in the 17-18 volt range.

One thing you need to know about batteries is that they all have internal resistance. That resistance varies depending upon many factors, one of which is charge level. As a battery becomes discharged, a healthy battery will readily accept a charge and internal resistance will be relatively low. As it charges up, internal resistance to the charge increases which will require progressively higher voltages to charge it. An unregulated supply might have the ability to charge it up if it has the ability to supply high enough voltages.

Which leads us to chargers. A battery charger is more than just a power supply. It also includes circuitry that, at the minimum, regulates the output. For example, if you wish a 2A charge level, the charger will ramp up the voltage to a level necessary to charge at the 2A rate. Since it is regulated, it provides the necessary control of the voltage and current to charge the battery at the optimum rate and charge level. Most chargers also include other monitors (for example, battery temperature) to factor into the charge. In any event, more sophisticated chargers, when it determines that the battery is fully charged, then switches to a trickle/maintenance mode.

Bottom line is that these boomboxes use SLA batteries because they are relatively cheap but they actually aren't the best choices for portable devices. NiMH or Lithium Ion are light years away better. Additionally, the chargers probably aren't even chargers, as you can see. Maybe they are just unregulated power supplies.

About resting voltage, 12 or 12.6 volts are common resting voltages for LA batteries. The other higher voltages you mentioned are not resting voltages. It is something known as surface charge or something like that which is not a true sustainable voltage. Batteries, after a charging session will have elevated voltage readings which will level out if allowed to rest for several hours, or after being put to use for a short time. At that point, the voltage drops to it's "true" resting voltage which it will remain until the charge begins to draw down.

Last thing.... you keep mentioning diodes. While it is generally well known that diodes can act like one-way electrical valves, you should know that diodes have another characteristic which is forward voltage drop. Typically this is in the area of .6 to .7 volts. So if you insert one diode into a circuit, the voltage will drop that amount. If you have 2 diodes in a circuit, the voltage will drop even more.

I know you don't want to deface your boombox but if I were you, I would simply wire the battery to some banana jack terminal or other external screw terminals. A proper battery charger can then be easily connected externally and provide a proper charge. Those jacks are not at all unattractive if you find the right ones, in fact, it can give the box a more serious appeal. Of course those jacks can be mounted on the side or back. But that's just me. Don't fear to put this box to use, I hate to say it but these newer technology boxes aren't meant to last forever. Once something internally fails, it's time to either mod it or toss it. They won't likely be repairable.
 

Transistorized

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Much appreciated superduper. I ended up installing a jack on the front. I have now relocated it away from the bass ports. Like you said, it actually doesn't look bad. I purchased a smart charger which is a 3 stage unit. Towards the end it goes into "float" stage and goes to as low as 100mA on the charge and it maintains it at 13.35.

This has worked very well. The box now goes from 3 to 4 hours before the first battery indicator bulb starts to go out. These mods were the best things I could have done. I recommend everyone with these boxes (even if they seem to be functioning correctly) to go ahead and get a proper SLA smart charger and connect an external jack. I do not believe the stock charging systems are truly topping off the batteries correctly. Users might actually notice more play time with a better 3 party charger :-)
 
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