# How to get 29.4V constant current from a 12V power supply?

I have a lithium battery that I currently have to charge using my lab PSU which I find rather annoying.
I'd like to use my 12V 24A PSU instead. But I need exactly 29.4V and the current must not exceed 1.5A.
I have a couple of step up converters, for example this one. But I would have no way of limiting the current to 1.5A with that or do I?

What would be a cheap way to do this? Are the any \$2 modules that could do that?

You say constant current but mention a voltage...

But yeah, you would need a constant current, constant voltage circuit. But 1,5A @ 29,4V = 3,7A @ 12V. So you already need a pretty large converter. With China ratings I would say 5A.

And yeah, you could make one with China modules but it will not be to simple. Simples way I can think of is:

• Get a boost converter (>=5A) set to 29,4V
• Get a step down converter (>=4A)
• Take off it's pot
• replace it with a current sense resistor in a way that 1,5A gives you Vref of the step down (so lower Vref is better) Vref / 1,5A = Rsense.
• Connect the step down to the step up

felic:
What would be a cheap way to do this? Are the any \$2 modules that could do that?

Sounds like your charging a large 7S LiPo ?

Myself the last thing I would suggest is using a 'cheap' way of doing this, I would use a charger designed for the job, not something costing \$2.

septillion:
You say constant current but mention a voltage...

I just meant that the max voltage should be 29.4V. While charging a Lithium battery, at first it charges at max current, but the voltage will be well below the maximum. Over time the voltage rises until it reaches the maximum and then the current will slowly go down until it's 0A. At that point the battery is fully charged.

Anyway, after spending some more hours on aliexpress going through almost every dc dc converter, I finally found a few promising ones:

LM2596S DC-DC LM2577S Step Up Down Boost Buck Voltage Power Converter Module Non-isolated Constant Current Board 15W 3A

DC DC Boost Converter Voltage Regulator CC CV DC-DC Step Up 10-30V to 12-35V 150W 10A Power Supply Driver Charger Adjustable DIY

Original 5A DC to DC CC CV Lithium Battery Step down Charging Board Led Power Converter Lithium Charger Step Down Module

Are these any good?

felic:
Are these any good?

Depends on what you want to end up with. A fully charged battery, or an explosion. If it's the second, sure, go ahead. Do make sure you won't hurt other people, animals, or property not yours. if it's the first, get a proper ready-made charger.

Are you aware that you need to monitor each individual cell of your LiPo battery during the charge process to ensure you do not exceed the maximum charge voltage on each cell.

LiPo chargers are far more complex than you seem to believe.

I just wanted to know if these step up converters could do what I asked for in the first post. Nothing will explode.

Step-up converters are voltage sources, delivering a constant voltage regardless of the current (within device limits, of course, and I'm idealising as in the real world voltage is usually not perfectly stable).

If you want no more than 1.5A coming out, you have to add a current limiting circuit to the output, but the moment the current limiting circuit kicks in, the overall output voltage will of course have to drop (Ohm's Law).

Lithium Ion batteries are no joke.
If you have a proper balancing board as part of the pack you shouldn't need to worry about the amps at all. More volts and amp( to a degree) would be better as it would allow the board to handle all of the heavy lifting of cell balancing and charging quickly , just give the board the voltage it needs and it will handle the charging duties handily.

I have built a few LiIon packs and never considered building one without balance charging. The biggest pack to date for me has been 7s8p . they were the super cheap 18650's you could get on ebay 4 years ago for <\$1 each. There were several instant duds in the batch, and several that became duds after 1-2 cycles.

29vdc on LiIon is an explosion and fire waiting to happen in a big way. I have never experienced that.

The dc dc converters that I linked in this post are marketed with constant current that's why ask.

Again, I don't worry about the battery exploding. I just don't want to charge it with more than 1.5A because that's the max according to the specs. Also, charging faster isn't better. There are several drawbacks.

I don't know if the battery does balance charging, but it definitely has a BMS and some protection features.

The bms should balance the pack. The bms should also limit charge/discharge current to the batteries design specs. Just because 10 amps of power are available doesn't mean the bms will allow that into the battery. It should be continuously monitoring charge/discharge state and adjusting accordingly, and shutting power flow off once the battery is fully charged or depleted.

Consider this. My battery in my car has 880Cca at 12v my headlight uses 55w@ 12v. When my headlight is turned on the light essentially shorts its terminals across the battery terminals(there is a fuse of course, but it doesn't limit current during normal operation) so that headlight bulb uses ~4.58 amps. Why isn't it getting burned out when exposed to 880 Amps? It's designed to only consume 4.58 amps, that's why.

If your battery isn't a super cheap cobble job, the bms should be limiting the current flow into and out of the battery and protecting it.

Post a link to the retail site for the battery. Then we can tell you if charging from a cc-cv supply is a good idea or not. Or if you need something like a hobby charger designed specifically for charging dubiously protected LiIon batteries.

Even a picture of the label on the battery would be really helpful.

Okay sure, I'll charge the battery with way more current than it is rated for. That sounds like a fantastic idea. Surely this won't burn down my house.
Honestly, I'm sure you want to help, but this is terrible advice and you are probably not an engineer. There is a good chance that the max charging current has a good reason. More current means more heat. More heat on everything: The wires, the PCB, the battery and everything surrounding it. What if the battery is meant to be used close to heat sensitive components?
Also, if a battery that is rated to be safely charged at a given current in a 60°C environment then it may just explode when charged at 3x the current in that environment, even though that current may be safe in a 45°C environment. More current also means faster capacity degradation and I definitely wouldn't want that.

Anyway, I ordered a cc/cv dc-dc converter now and I'll just go with that, hoping that it works.

If the bms used with your battery is designed for your battery it will limit charge current. Of ypu are concerned of over current you can easily figure it out.

Put a dvm on amp mode and put it between the charger and battery. See what the battery draws. If it's more than 1.5A you know the BMS is only half assed and only cuts charge/discharge at voltages and doesn't limit current.

I have had to mod bms for packs I've made to limit current both in and out.

Consider this, my cellphones max charge rate is 7watts, why can I safely plug mt cellphone into a 25watt supply? My cellphone has a built in charge/discharge controller(bms) that limits current.

Just sayin.