Looking for an internal battery charger

Jack.Straw

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Hello. Over the years i've been building little amplifiers for friends. I generally use 8x AA NiMH batteries to power them. It's important to me that the user not have to remove the batteries in order to charge them. It's also important to me that they be able to use the amp while the batteries charge. In the past i've always built my own battery charging circuit using the Maxim MAX712 or MAX713 (i can't remember which atm). They work great, and deliver exactly the functionality i want. However, i do it all on perfboard and i really hate the process of building the chargers. People are always asking me to make them an amp, and i find myself turning them down simply because i don't feel like making another charging circuit.

I've experimented with using a couple different external chargers. I've used the Tenergy Smart NiMH charger, and also the type where the charging circuit is built right into the wall-pack. Both of these work just fine for charging the amp, but neither allows the amp to be used while charging is in process. Also, they charge considerably slower than the Mixim chip.

So, does anyone know of a pre-built charging circuit that would suit my requirements? Perhaps i'm just not using the correct search terms, but i'm finding nothing of the sort available. Alternatively, if i can't find something pre-made, does anyone know of a printed PCB for the Mixim chip (or another NiMH charging chip really, i'm not that picky..) that i could use?

Thank you for your time and any advice you can offer!
-Scott
 

CuriousOne

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Hobby chargers might do the task, but not sure.
I'm also building amplifiers, but I go different way - I use single Li-Ion cell, which is charged via micro usb, and use step-up converter to boost voltage to 12V, so amp can operate nicely.
 

Jack.Straw

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CuriousOne, thanks for the response! I'd be interested in knowing more about the Li-Ion cell and the amp you use. Is it a 12v amp? What kind of battery life are you seeing? I'm not married to the NiMH, it's just what a chose about 5 years ago after doing a little web research. They work great, but if Li-Ion delivers a similar battery life I wouldn't mind exploring that option..

Thanks again,
-Scott
 

CuriousOne

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I'm mostly using Class D PAM8610 chip based amps. It works from around 7 to 15 volts, and can deliver 2x10W into 8 ohm load and 2x15W into 4 ohm load.

For step-up, I'm using Linear Technology's LT1370 chip. It can deliver 2amps output at 12V, at 3.6V in (standard lithium cell voltage).

For the cells, I'm using protected Panasonic NCR18650B's in parallel, 4 cells total. I haven't measured battery runtime, but I know it is well over 12 hours, if not 20 hours (you know, load depends on music type, too).

For charging, I'm using cheap TP4056 boards available on ebay, two of them in parallel, to deliver 2 amp charge current. Full charge of above cells takes approx. 8 hours I guess - I just plug them in the evening, and unplug charged in the morning.

Main advice: Do not waste your money or time on cheap step-up boards from ebay, which claim fantastic efficiency, even showing some scope and power supply connected.

Also, NiMH time is over long time ago. LiIon delivers up to 5x more capacity for the same weight and price, too.

Let's take 4 pcs of 2000mAh eneloops. They deliver roughly 1.2x4x2=9.6wh power and cost $14. For the same price, you can get 4x 3.7v 3400mah Panasonic cells. They deliver 3.7x4x3.4=50.32wh power. So basically, you're getting 5x more runtime for the same money. One must is that lithiums need protection, deep discharge or overcharge will cause very bad things.
 

Jack.Straw

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Awesome, thanks CO! I'm going to try out your route and see what kind of results i get. Thank you so much for taking the time to share the info!
 

Gauss163

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... Main advice: Do not waste your money or time on cheap step-up boards from ebay, which claim fantastic efficiency, even showing some scope and power supply connected...

I don't think the answer is that simple in general. For example, all of the cheap ones I have do meet their efficiency claims (not difficult nowadays). But they may have other issues that impact certain apps. What issues did you encounter?
 

CuriousOne

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There are two main issues.

Very often, Chinese declare much lower lowest possible input voltage, than chip supports. For example, may boards, using XL6009 chip, often are declared as capable of working from 3V or 4V. Where 5V is absolute minimum, according to datasheet, and to get efficiency above 60%, you need to have input voltage at least 6-7 volts.

2nd: They in 99% case use fake ic. For example, if you measure "chinese" LM2596 IC frequency, you will find it close to 50khz, not to frequency of the real one. I've seen X-ray pictures of genuine LM2596 chip and chinese one - totally different inside.

And if you defend these boards, can you please suggest me the model of such step up board, which will output 12V 2A from 3V DC input?
 

CuriousOne

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And another issue. Often, chinese sellers, specify switch maximum current as output current. So, you will never get declared current out of that module.
 

Jack.Straw

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CuriousOne, i have some questions if you wouldn't mind helping me understand some things. I'm looking into going your route. I already have a PAM8610 amp laying around, so i can use that for my test.

For step-up, I'm using Linear Technology's LT1370 chip. It can deliver 2amps output at 12V, at 3.6V in (standard lithium cell voltage).

For the cells, I'm using protected Panasonic NCR18650B's in parallel, 4 cells total. I haven't measured battery runtime, but I know it is well over 12 hours, if not 20 hours (you know, load depends on music type, too).
I've always put my batteries in series to obtain the required voltage of the amp. I understand that this configuration keeps the same mAh as a single battery. Putting the batteries in parallel, you are quadrupling the mAh, but only delivering 3.6 volts. You then have to step-up the voltage from 3.6 to 12, which i (perhaps incorrectly) assume takes energy? Would you say that 4 batteries in parallel driving a step-up circuit is going to result in longer battery life than just putting them in series without the need for a step-up circuit?

For charging, I'm using cheap TP4056 boards available on ebay, two of them in parallel, to deliver 2 amp charge current. Full charge of above cells takes approx. 8 hours I guess - I just plug them in the evening, and unplug charged in the morning.
Am i correct in assuming that you cannot use the amp while charging is in process? Should the batteries start failing during use, i would like to be able to plug it in and keep using the amp... and have the batteries charging at the same time. That's how my old NiMH system worked and i'm a bit spoiled by that.

If the batteries were in series and you used one of the 4 cell Li-ion charging boards from ebay, would't the required input voltage for charging also be in the acceptable voltage range of the amp?

Sorry if i'm way off... my knowledge is obviously lacking... which is why i'm here :)

Thanks again!
 

CuriousOne

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The main reason, that I'm using so "complicated" setup, instead of simply connecting 4 cells in series, is the fact that, all my amplifiers are completed product, they are not designed batteries to be removable or serviceable by end user. Another reason of "single cell" mode selection is ease of charging - Instead of utilizing multi-wire balancing chargers with hard to learn settings, amplifiers and other portable gear made by me, can be charged by common MicroUSB cable.

So, if you going to use protected 18650 cells, and charge them in stand-alone charger, you can safely connect them in series. But I can't go that way - my customers are upper class people, they pay up to $5k for having simply to use, simply to operate, but luxurios gadgets (for example, I've recently made a 60s style vintage radio bluetooth speaker for a U.S. customer with Hindi origins - cabinet was CNC-ed from piece of sandalwood, goatskin leather was used for trimming, and all metallic surfaces were gold plated. Can you imagine such customer removing batteries for charging, setting up balance connection or any other high-tech servicings?)
 

HKJ

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Parallel makes it a bit easier to use multiple LiIon cells, if you use them in series you need a balancing BMS to keep optimal performance. This is not difficult to get, but it cost some money and means you need a charger for the actual voltage (This is not as common as a 4.2 volt charger).
I will not say one solution is superior to the other, it requires analysis in each case:
4.2 volt: easy to charger, no risk of unbalancing cells, but requires a boost converter.
Higher voltage: Requires a balancing BMS and a charger for the voltage, but has no looses to a boost converter.
 

CuriousOne

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It also depends on how you want to charge batteries. If charger is incorporated into device, then using cells in series is much easier. But imagine, you have 4 cells in series, and you want to use external charger, and you want to use single connector, instead of separate ones for balance and charge. You want everything to look tidy, compact and practical. You will need at least 6 pin connector. And, surprise surprise, pair of (M+F) good quality connectors from HiRose or Amphenol will cost you about $50 :)
 

Jack.Straw

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I've been trying to get a handle on all this and my head is starting to spin. More questions:

1) If a li-ion battery is listed as "protected", does that mean it has built in over-charge and over-discharge protection? In other words, if i buy protected batteries, does that mean i do not need a separate protector circuit like THIS ONE?

2) I'm finding lots of 4 cell charging circuits available like THIS ONE. If i'm looking at that correctly, it looks like it's meant for charging 4 batteries in series. However, if i'm understanding what you guys have said, then this sort of charger is not good because it does not keep the cells balanced? What about THIS ONE? That one seems to have a balancer, right?

3) Is passively charging these batteries an option? What i mean is, if i was to use protected cells and build a simple voltage regulator which would supply exactly 14.8 volts, wouldn't the cells charge and balance eventually on their own? So long as the input voltage never exceeds 14.8v, couldn't the batteries receive that input indefinitely with no ill effects? If that is true, then plugging the amp in could simultaneously charge the cells and power the amp?

Thanks guys.. i'm trying to wrap my brain around this
 

CuriousOne

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1) Mostly yes, you won't need separate protector, but read below for "mostly"

2) None of these are charning circuits. These are balance/protection circuits, so you can charge your series lithium battery pack with simple constant voltage/constant current chargers, like this one: http://www.ebay.com/itm/Lithium-Ion...-7-2V-7-4V-1A-Wall-Socket-AC-DC-/171789311448 Please note, these circuits also act as battery protectors, so you can use unprotected cells with them.

3), Yes, but not reccomended. Generally, lithium battery charge is considered to be complete when voltage reaches 4.2 (4.1, 4.35 - depends on battery) volts, and current thru the battery becomes 10 times less that was charging current. So say if you connect battery to such charger, it will be "topped over" with the time, and this is not healthy for lithium batteries at all.
 

HKJ

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A protected battery will protect itself from high or low voltages, but it will not do any balancing.
When looking at circuits you have to look at he abilities of the circuit:
1) Can it protect the batteries
2) Can it balance the batteries
3) Can it control voltage and current to charge the batteries, you selection in 2) cannot do that.


The correct solution is a protection and balancing circuit and a charge controller.

Without a balancing circuit the batteries may work for a year or two, but LiIon batteries are not self balancing and will get out of sync. This means less capacity and early reported failure.
 

CuriousOne

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The main difference between balancing and protection circuits can be formulated like this:

If overvoltage is detected, protection circuit turns off battery.
If overvoltage detected, balancing circuit acts as paralel shunt, and consumes extra energy, to maintain battery voltage close to 4.2 volts (or how much is needed)

For example, Sony camcorder and camera batteries, all have 2s protection/balancing circuit built in. Some even have fuel gauge (NP-xx D series.)
 

CuriousOne

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And oh yes, if you're familiar with MAX712/713, then get MAX745 - almost the same thing, but for lithiums :)
 
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