Let's assume I have a 12V 90Ah battery and 12V(15v max) 20A SMPS. I want to use this SMPS to charge the battery. I heard that 1/10 of the battery current is good to charge the battery. So that means 9A is the best current to charge the battery.

The matter is if I use the above SMPS will that SMPS reduce the current accordingly or give the total 20A current.

  • \$\begingroup\$ What SMPS is it? \$\endgroup\$ Commented May 14, 2022 at 7:42
  • \$\begingroup\$ Look up the maximum charge current in the battery's data sheet. 20A sounds like a bit much. A fixed voltage power supply is not a proper battery charger except maybe for keeping it at a float voltage of 13.5-13.8V. You would probably be better off with a proper 3-stage charger. BTW, you cannot charge a lead acid battery to just 12V. It will never completely charge (just 50% State of Charge) and will not last many cycles. \$\endgroup\$
    – StarCat
    Commented May 14, 2022 at 10:55
  • \$\begingroup\$ I am sorry 15V max meant that the SMPS has a preset to adjust the voltage from 9V to 14.8V I think 14.2V is the sutiable voltage to charge a 12V led acid battery isn't it? \$\endgroup\$
    – Harsha
    Commented May 15, 2022 at 9:07
  • \$\begingroup\$ IF SMPS can be set to current limit AND voltage limit (some can, some can't) then set to 9A and 14.2V. It will limit at 9A when VBat < 14.2V (Constant Current mode = CC) as battery charges and then when Vbat = 14.2V it will change to constat voltage mode (CV) and current will slowly reduce. This is OK to mostly charge your battery but lead acid needs a boost or topping charge to fully charge. See eg battery university for more information OR many related answers on this site. || If the supply has no current limit control this is not a suitable method. \$\endgroup\$
    – Russell McMahon
    Commented May 15, 2022 at 11:58
  • \$\begingroup\$ @Harsha, SMPS seems like the metal cased ones with a small trimpot for fine voltage adjustments. These SMPS do not have current limit, just overcurrent protection, sometimes triggered at 120%~150% of Max. current. Further, this protection is just “hiccup” like: If protection triggers, it powers down completely for ~1s, then tries again, can trip again, and so on. Also, a good 90Ah can have an internal resistance 6~10milli Ohm. Depending of battery’s SoC a voltage difference (SMPS-Batt) as 500mV is expected, but battery would try to drain more than 30A, tripping the hiccup protection. \$\endgroup\$
    – EJE
    Commented May 15, 2022 at 15:47

2 Answers 2


No, because a switch mode power supply is not a battery charger.

Power supply can also be damaged when connected to another voltage source, the battery.

You need a battery charger to charge batteries.

Power supplies output constant voltage such as 12V, so it can't be connected to an empty battery with for example 11V voltage. If the battery has internal resistance of 0.05 ohms, the charging current would already be 20 amps, and if the voltage or resistance are lower, the current will be higher, and the power supply can't handle that and will hopefully detect overcurrent or undervoltage condition, and shut down.

Even if it did charge the battery, it would charge the battery to 12V, so it would not be the voltage the lead acid battery should be charged with.

  • \$\begingroup\$ I assume by "15v max" it's slightly adjustable, so could be set to the lead acid charging level (13.8-14.4V or so, depending on exact type). Current limiting is usually provided on most supplies -- but that is something that should be confirmed first. \$\endgroup\$ Commented May 14, 2022 at 7:42
  • \$\begingroup\$ @TimWilliams Power supplies generally don't offer current limiting, but overcurrent protection. So in this case a 20A 12V supply would push up to 20A because battery voltage of 11V does not allow voltage to rise to 12V and exceeding 20A would shut it down, and maybe restart only to shut down again so it is in hiccup mode. So not normal operation for a power supply. \$\endgroup\$
    – Justme
    Commented May 14, 2022 at 10:41
  • \$\begingroup\$ Depends on the type; my designs have both, but of course not everyone does. Some common types do, like flyback, or current mode forward. So, it depends. \$\endgroup\$ Commented May 14, 2022 at 11:10
  • \$\begingroup\$ @TimWilliams Indeed, and the question was hypothetical anyway, and batteries still are best charged with a device intended to charge said type of batteries to make best use of them. \$\endgroup\$
    – Justme
    Commented May 14, 2022 at 11:17
  • \$\begingroup\$ @TimWilliams yes it is adjustable SMPS which is having a preset to adjust from 9V to 14.8V but the current is not adjustable thats the concern \$\endgroup\$
    – Harsha
    Commented May 15, 2022 at 9:11

Your question:

If I use the above SMPS will that SMPS reduce the current accordingly or give the total 20A current?

Short Answer: Most probably it will not provide an adequate current, triggering the overcurrent protection at 25A~30A.
So, the charging current will be erratic and the setup is operating in a mode not recommended for the battery and for the PS, which could wear out the battery and/or damage the SMPS.

Detailed Answer:
Most probably the mentioned SMPS is like the following collage, where this webpage registers a detailed reverse-engineering of a similar 12V 33A SMPS.

enter image description here

The adjusted voltage in the SMPS needs to be higher than a discharged battery, where this differential should be adjusted to at least 0.5V ~ 1.0V using the trimpot (green markings).
A 12V 90AH battery can have an internal resistance of 6 milli Ohm (mR) - as here. Even assuming 3 times as high as R_int = 20mR, the current would be 50A for deltaV= 1.0V.

Outcome: the circuit of a 20A rated SMPS will repetitively trip-off the overcurrent protection, about every second.
Somehow this self-restart feature is nicknamed as “hiccup” protection, and is often seen in several SMPS (in TVs, etc.).
The control IC used in most of these SMPS is the TL494 as didactically seen in Figure 9 (or similar IC - red markings). The red highlight below shows how that 12V 33A implemented its protection:

enter image description here

An unspoken question:

How can someone make this kind of SMPS to be able charging a lead-acid battery?
The following setup approaches the recommended settings for a 2-stage Lead-Acid Charger:

  1. Use a small-value and high-power resistance; the use of 2…4 headlight lamps in parallel (each 12V 50~60W) can create an inexpensive power resistor. Use this “resistor” in series between the SMPS and the L.A. battery.
    Each lamp will limit the current at 4A max. and present a cold resistance about 10% of the incandescent filament value: R_1V ~= 300mR.
    Filament’s intrinsic non-linearity will make a poor-man’s current limit. For 12x the voltage variation (12V/1V) the current varies just 4 times, as further referenced here - incandescent lamp ratios.
  2. Adjust PS voltage to deliver V_charge = 14.2V, necessary for topping the charge - see BU-403 here. Charge for 8~16h and
  3. Monitor the battery voltage, after leaving it to rest for 30min~1h.
  4. This V_charge = 14.2V is not to be left unattended or indefinitely - limit it to about 24h. A floating voltage would be like V_float = 13.5V.

There is a post better explaining this battery, lamp, and battery-charger interaction, including Cold-Hot filament resistance measurements (plotted with data) of another 12V car lamp.

About Floating voltage: The final stage of a classic 3-stage SLA battery charger does the voltage switch-over from V_charge = 14.2V to V_float = 13.5V automatically, based on the charging current and/or an internal timer.
You could be mimicking this reduced-voltage from V_charge by using the Voltage drop of a silicon diode: 0.7V ~ 1.0V at its rated current.
Using a time-controlled relay and switching from V_charge = 14.2V to V_reduced = 13.5V ~ 13.2V, would complete this impromptu exercise to ”convert” a SMPS into a workable battery charger.


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