0
\$\begingroup\$

I am trying to make a circuit that charges a medium size lithium 12V battery (4500mAh) it draws power from a car's battery/alternator. When my car turns on I have a simple circuit that closes the normally open connection between the Vin of the charging circuit and the car’s voltage supply line after some delay.

Once connected to the car’s voltage line the charging circuit needs to charge the auxillary battery to 12.6V and not allow a current flow greater than 300mA. I took these specs from the battery manufacturer, I attached what I could dig up below. I decided to use the LM317 linear voltage regulator, an adjustable voltage regulator, to supply voltage to my battery.

But I am realizing that I can’t linearly scale the car’s voltage rail because it varies from say 13-14V and if I am targeting 12.6V on the Vout unless I dynamically adjust the variable resistor, I cannot maintain 12.6V on the outlet. I do understand that there are such things as fixed linear voltage regulators but here too lies a problem, finding one that is fixed to 12.6V is not so easy although finding one that is fixed to 12V is…

Another option is to regulate the input voltage to a higher value say 24V, before feeding it to the LM317 but I am not sure if this is a good idea either.

circuit

battery

voltage regulator sheet

edit I was searching through some LDO regulators and it appears not all regulators float their Vout according to their Vin for example I have seen several that in ADM7170 the outlet voltage appears to be independent of inlet voltage (in a predefined range) and for the configuration below voltage out of the regulator can be found using the equation below... Is it untrue what I posted above that adjustable regulators outlet voltages vary with inlet voltage?

LDO example

\$\endgroup\$

2 Answers 2

Reset to default
1
\$\begingroup\$

You can get a SEPIC (Buck/Boost) module for a few dollars. Some of them are designed to charge batteries. Ou you can use that SEPIC converter to power your Lithium charger. The charging profile for lithium battery is different then a normal lead acid battery. Using the lead acid profile your battery will be fairly discharged before the charger turns on and starts charging. A lithium charger will do a good job. Good Luck!

\$\endgroup\$
1
  • \$\begingroup\$ I believe this SEPIC term is what I am looking for, reading about some modules now. \$\endgroup\$
    – Feynman137
    Commented Jul 13, 2022 at 1:23
1
\$\begingroup\$

If you take a look at this datasheet it says that the recommended minimum input-to-output differential voltage for the LM317 is 3 V. This means you need at least 15.6 V at the input for an output of 12.6 V.

I missed that you posted a datasheet, in that one it looks like it will be 2 V at 25 \$ ^{\circ} \$C and 300mA. So, still 14.6 V minimum.

You might be able to find a low dropout (LDO) regulator that will work, or use a buck-boost converter.

\$\endgroup\$
1
  • \$\begingroup\$ I agree an LDO is a good idea, because input and output voltages are so close. But I would still be using an adjustable LDO which would make the output voltage move with the input voltage (my problem). I think the boost converter is a good idea am exploring this currently \$\endgroup\$
    – Feynman137
    Commented Jul 13, 2022 at 1:10

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.