Can I use boost/buck circuit board as a 12v lead acid charger?

Thread Starter

zirconx

Joined Mar 10, 2010
145
Hi, I'd like to maintenance charge my Ford chassis battery from my motorhome electrical system. This requires a DC-to-DC charger, which are typically expensive. Could a cheap buck/boost board work for this? Like this one? https://www.ebay.com/itm/262693647349

I'm guessing the battery might draw too much current if/when it's discharged and kill the board?

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Item Description:

Modules nature: non-isolated Boost Buck (SEPIC)
Rectification: Non-synchronous rectification
Input voltage: DC 3-30V
Output voltage: DC 0.5-30V
Input current: 3A (MAX)
Power: 25W (MAX)
Conversion efficiency: 95% (MAX)
Switching frequency: 150KHz
Output ripple: 40mV (max)
Load regulation: ± 0.5%
Voltage regulation: ± 2.5%
Operating Temperature: -40 ° C to +85 ° C
Dimension: 48 x 23 x 14 mm
Output short circuit protection: Yes.( If short circuit, the output will be 0, until exclude short-circuit )
Typical output, do not need heatsink (reference):

Input 12V----Output 12V/2A; 24W
Input 12V----Output 5V/3A; 15W
Input 12V----Output 13.8V/1.8A; 25W
Input 5V----Output 3.3V/3A; 9.9W
 

crutschow

Joined Mar 14, 2008
28,551
How discharged will the battery be when you want to charge it?
Do you want to leave the battery continuously on the charger?
What is providing power to the motor home electrical system?
 

Juhahoo

Joined Jun 3, 2019
257
Most probably not. If this is typical SMPS converter, it wants to maintain the output voltage at a certain level. If your battery is empty, it will be seen as a short circuit for the supply. The power supply needs to be constant current (+constant voltage) regulator which will maintain its output current up till its set max point and once the battery voltage reaches the set value, output current is reduced. Surely you could use series resistor, but charge time gets very slow.
Learn more by Google: "constant current SMPS" or "constant current battery charging"

one example:
https://www.ebay.com/itm/3535532545...XHQ0LsCBRS_bitxdSldsNotqKfrVLpYhoCE7QQAvD_BwE
 

Thread Starter

zirconx

Joined Mar 10, 2010
145
A set of Jumper-Cables would be easier.
There is no need for any Electronic Controls.
I would need a very long set of jumper cables (30ft maybe), and I don't want to have to open both battery compartments and hook up each time. Maybe by jumper cables you meant simply wiring the two batteries together. This cannot be done for a number of reasons. They are separate systems and have different battery chemistries (the motorhome is lithium iron phosphate, the Ford chassis is lead acid). My goal is just to maintenance charge while I camp or while I am parked for storage.

How discharged will the battery be when you want to charge it?
Do you want to leave the battery continuously on the charger?
What is providing power to the motor home electrical system?
The chassis battery will be full when I park it (either for camping or storage). I would like to maintain it, to combat against parasitic drain and internal drain.

I would like to be able to leave it continuously connected. I would probably install a switch so I could turn it off when the motorhome has limited power.

The motorhome will be powered by either
1) Plugging into to shore power - in that case an onboard lithium specific charger charges the house lithium battery and provides power for all 12v appliances.
2) Solar + generator as necessary. The generator powers the above mentioned lithium battery charger.

Currently the motorhome is in storage and connected to shore power. So I am using a battery tender plugged into one of the 120v RV outlets, and the output is plugged into the cigarette ligher plug on the Ford chassis (mine is not switched with the key). Of course this only works when plugged into shore power, so I am investigating a DC solution which would work on both shore power or solar/generator power.

Most probably not. If this is typical SMPS converter, it wants to maintain the output voltage at a certain level. If your battery is empty, it will be seen as a short circuit for the supply. The power supply needs to be constant current (+constant voltage) regulator which will maintain its output current up till its set max point and once the battery voltage reaches the set value, output current is reduced. Surely you could use series resistor, but charge time gets very slow.
Learn more by Google: "constant current SMPS" or "constant current battery charging"
If the battery is empty, and the power supply sees it as a short circuit - that would be fine with me as long as the board shuts down and doesn't burn up. I don't need it to be able to recharge a dead battery. Looking at the specs on the one I posted, I see it does say Output short circuit protection: Yes.( If short circuit, the output will be 0, until exclude short-circuit ).

I will try to understand your point about constant current charging. I thought that for float charging you could use a constant voltage supply.
 

LowQCab

Joined Nov 6, 2012
1,619
This LDO-Linear-Regulator can be configured for virtually any Voltage or Current Limit,
Limited only by the rating of the chosen MOSFET.

It can also be used as a Power-Switch.

The required Heat-Sink is the most expensive Part,
and its size will be dependent upon the Maximum-Current that You set,
which will determine the worst-case Power-Dissipation of the FET.

The Current-Limit setting must not be greater than
the Amperage-Capacity of the gauge of wire that You intend to use.

If You will state your maximum desired Charging-Current,
I'll make some suggestions for a MOSFET and an appropriate Heat-Sink.

For size and cost practicality, the Current-Limit should be less than around ~30-Amps.

While this Circuit will work just fine with a Current-Limit of just ~1-Amp,
there are simpler ways to achieve a very low-Current Float-Charge.
.
.
.
Extreme Current Linear Regulator .PNG
 

Attachments

Ian0

Joined Aug 7, 2020
4,145
I will try to understand your point about constant current charging. I thought that for float charging you could use a constant voltage supply.
Correct about float charging, but before you get to float charging, you have to do bulk charging, which is constant current.
 

crutschow

Joined Mar 14, 2008
28,551
You do need a buck-boost converter since the float-charge voltage may be above the motorhome battery voltage.
The linear regulator suggested by LQC thus will not work under those conditions.

If you can set the DC-DC converter to 13.4-13.5V output, that should provide a proper trickle-charge and maintain the vehicle battery at full charge.
I don't see any problem with high current if the vehicle battery is fully charged when you connect the converter and if it safely current limits.
 

Thread Starter

zirconx

Joined Mar 10, 2010
145
...
For size and cost practicality, the Current-Limit should be less than around ~30-Amps.

While this Circuit will work just fine with a Current-Limit of just ~1-Amp,
there are simpler ways to achieve a very low-Current Float-Charge.
...
Thank you. I think 500ma will be enough to maintain the battery.


Correct about float charging, but before you get to float charging, you have to do bulk charging, which is constant current.
But I plan to start fully charged. I will have driven to my camp site or the storage location - the battery should be fully charged at that point. I don't need a charger to do bulk charging.
 

Juhahoo

Joined Jun 3, 2019
257
I don't need it to be able to recharge a dead battery. Looking at the specs on the one I posted, I see it does say Output short circuit protection: Yes.( If short circuit, the output will be 0, until exclude short-circuit ).

I will try to understand your point about constant current charging. I thought that for float charging you could use a constant voltage supply.
Just test what it does when output is shorted. I still would choose a type that limits the current instead of shutting it completely off.
 
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