# How to make 12V lead acid battery last longer by dropping voltage with a resistor?

I came across one of those Fisher Price kid cars in a sorry broken state. I took it apart, redid all the wiring for 12V and dropped in a sealed 12V Lead Acid 7Ah battery in it. It works great. Lasts about 15 minutes. The original design was for 6V battery, so it goes fast. I am thinking about dropping the voltage to somewhere in the 7V to 9V range to make it last longer yet not be super slow.

The only way I know how to drop voltage is either via voltage divider (2 resistors), or the way I drop voltage from 12V for LED to not burn out via adding a single 1KΩ resistor to the LED's leg. I also know I could use voltage divider or regulator, but I dont need this to be precise voltage, anything between 6V and 12V is fine. Id like to make the simplest circuit possible and hopefully with tools I already have.

I have several questions/worries:

1. My worry is that this wont make the car last much longer as the power will simply be turned into heat in the resistor. Or is it reasonable to expect that if at 12V it lasts 15 minutes, at 6V it will last closer to ~30min?

2. If this is a viable way to go, do I need some sort of oversized resistor? I have a kit that is rated for 250V but 1/4W power rating. Im guessing that wont work? I also have a single 1Watt rated 100Ω resistor. I know that as it is the car pulls somewhere between 20 and 30Amps at peak. Does this mean I need 30*12 = 360 Watt rated resistor? Or does that math work differently?

This is the current setup (there are 2 motors 1 for each back wheel). I was thinking about simply adding (need to figure out value) a resistor before or after the 30A fuse.

1. And sort of a side question. Am I understanding correctly that even if motors do not draw any power, the resistor in line with LED constantly drawing power and converting it to heat? Im dropping down 12V to sub 3V with the 1kΩ resistor. Is the actual usage as far as the battery is concerted 12V by that LED? or is it draining at significantly lower rate? Than for example if I had some 12V LED and hooked it up the same without any resistors? Or is it exactly the same usage 3V LED with resistor and 12V LED without resistor?

Thanks!

• You should be able to find a suitable 6 volt battery - this would be much simpler than using a DC-DC converter to reduce the voltage (if you could find one to handle the motor current). A resistor won't work well, as the voltage it drops will depend on the motor current. Commented Oct 15, 2022 at 4:28
• I was given a kid's ride-on tractor that had two 12V 555 DC motors, one for each wheel, and a high/low speed control wired them in series for low speed and parallel for high speed. But it would only run for less than a minute before the thermal fuse opened. I found that the motors drew 5-10 amps under no load, and one was worse than the other. I bought replacement motors and they ran nicely with only about 100 mA no load. There was nothing obviously wrong with the motors. You might also try a series/parallel connection, but a PWM speed control would be best. Commented Oct 15, 2022 at 4:30
• @PStechPaul how to achieve PWM speed control? what components are needed?
– Duxa
Commented Oct 15, 2022 at 5:13
• Search for "pwm dc motor controller" on eBay or Amazon. Plenty of choices for $10 or so. Commented Oct 15, 2022 at 5:23 • @PStechPaul - A PWM controller seems to be another good way to efficiently use a 12V battery with a 6V motor. At a 50% duty cycle the average current might be close to that of using a 6V battery. The PWM unit would need to be capable of the high current draw of the electric motors (at 12V input). The motor drive pedal is likely just a simple on off switch so it might be worth implementing a switch to serve as sort of a fast/slow option, each position gives a different PWM %. As with a step down power supply PWM controllers can be purchased as complete modules (from Amazon, etc,). – Nedd Commented Oct 15, 2022 at 5:23 ## 1 Answer Using a resistor to drop the voltage is just reducing the voltage by way of wasted heat dissipation. The best way would be to find a DC/DC step down module that has a high efficiency. These are often call switch mode DC/DC converters. Depending on the efficiency you can then enjoy nearly the full watt hour rating of the battery. You would need to estimate the full load current to find a module that meets or exceeds that current requirement. You can find these on Amazon and even Wallmart.com. (Here's just one example: https://www.walmart.com/ip/DC-12V-24V-Step-Down-to-DC-6V-10A-60W-Waterproof-Power-Converter-Regulator-DC-DC/122710047 ) The LED question is very similar, but on a much less significant level. The series resistor simply drops the voltage by dissipating the extra power as heat. For example, with a 12V battery, a typical 2V LED, and the 1K resistor (as you show), the LED would use 20mw, (2V x 10mA), while the resistor will dissipate (or waste) 100mw, (10V x 10mA). • Thanks for the answer. I take it, there is no other way to limit the power other than drop voltage? Maybe some way to make motors draw less? I wonder how the official Fisher Price ones last (as claimed) over an hour with 12v 9Ah batteries.... – Duxa Commented Oct 15, 2022 at 5:07 • A 12V 9 Ah SLA battery should give about 1 hour of run time with a 5 amp average load, due to the Peukert effect. Your motors may be worn out like the ones in my kid's tractor. Good motors should draw only 100 mA or so under no load. If your motor runs just 15 minutes, the load is probably 16 amps or so. See my Peukert calculator: enginuitysystems.com/EVCalculator.htm Commented Oct 15, 2022 at 5:20 • @PStechPaul what "wears out" on motors? Is it repairable? Also, these motors have zero markings, how do I know what to look for if I want to replace them? – Duxa Commented Oct 15, 2022 at 5:56 • There are many "standard" DC motors for toys, robotics, hobbies, and small power tools. Here is a website with a wide assortment. You can measure the diameter, length, and shaft size, and choose from operating voltage and RPM and torque. I was unable to repair the motors in the kid's tractor. Probably failing winding insulation. kinmore.com/product/pmdc-motor/… Commented Oct 15, 2022 at 6:26 • I ended up buying the$10 PWM motor controller from ebay. Works well, on lower end makes the battery last about an hour and 30... somewhere in the middle lasts about an hour. Thanks all.
– Duxa
Commented Oct 25, 2022 at 5:27