# Can a 1 amp trickle charger fully charge a car battery? **URL:** \ **Category:** Factual Questions\ **Created:** [May 27, 2020, 7:22pm UTC](https://boards.straightdope.com/t/can-a-1-amp-trickle-charger-fully-charge-a-car-battery/854560 "2020-05-27T19:22:47Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![Squallous](https://avatars.discourse-cdn.com/v4/letter/s/22d042/32.png) [@Squallous](https://boards.straightdope.com/u/Squallous)\ **Post date:** [May 27, 2020, 7:22pm UTC](https://boards.straightdope.com/t/can-a-1-amp-trickle-charger-fully-charge-a-car-battery/854560/1 "2020-05-27T19:22:47Z")
Presuming that the car battery is otherwise good, functional, holds a charge, etc. , in theory a 1 amp trickle charger (battery tender, drip charger) intended for a motorcycle battery should be able to fully charge a car battery, right? Slowly, of course, but over the course of 48 hours it should be able to top it off, right? The charger has one indicator light: flashing amber = not charging, solid amber = charging, flashing green = 80% charged, solid green = full charge. It’s been connected to the car battery for 48 hours and showing solid amber this whole time. Thanks. ---
**Author:** ![dougrb](https://avatars.discourse-cdn.com/v4/letter/d/76d3ee/32.png) [@dougrb](https://boards.straightdope.com/u/dougrb)\ **Post date:** [May 27, 2020, 8:43pm UTC](https://boards.straightdope.com/t/can-a-1-amp-trickle-charger-fully-charge-a-car-battery/854560/2 "2020-05-27T20:43:32Z")
I’ve charged car batteries from a variable voltage power supply I built in high school 40 years ago and it can only put out 0.5 amps. It can take a long time. ---
**Author:** ![QuickSilver](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/quicksilver/32/7832_2.png) [@QuickSilver](https://boards.straightdope.com/u/QuickSilver)\ **Post date:** [May 27, 2020, 8:52pm UTC](https://boards.straightdope.com/t/can-a-1-amp-trickle-charger-fully-charge-a-car-battery/854560/3 "2020-05-27T20:52:26Z")
> [@Squallous](#): > > Presuming that the car battery is otherwise good, functional, holds a charge, etc. , in theory a 1 amp trickle charger (battery tender, drip charger) intended for a motorcycle battery should be able to fully charge a car battery, right? Slowly, of course, but over the course of 48 hours it should be able to top it off, right? > > The charger has one indicator light: flashing amber = not charging, solid amber = charging, flashing green = 80% charged, solid green = full charge. It’s been connected to the car battery for 48 hours and showing solid amber this whole time. > > Thanks. It should. Two things may be going on: 1) Charger is faulty/defective, 2) Assumption about battery being good is incorrect. Have you tried putting a voltage meter across the terminals with the charger plugged in and the charger unplugged (just the battery)? What does it read? A good battery should be anywhere from 12.3V to 12.7V. With the trickle charger plugged in, it should read between 13.2V to 13.7V. Obviously, the trickle charger will cycle on and off when the battery reaches full charge. Disclaimer: IME ---
**Author:** ![jnglmassiv](https://avatars.discourse-cdn.com/v4/letter/j/f07891/32.png) [@jnglmassiv](https://boards.straightdope.com/u/jnglmassiv)\ **Post date:** [May 27, 2020, 9:06pm UTC](https://boards.straightdope.com/t/can-a-1-amp-trickle-charger-fully-charge-a-car-battery/854560/4 "2020-05-27T21:06:05Z")
Regular car batteries are usually from a few dozen to perhaps 100 amp-hours. If the battery is in good shape and taken from a well maintained vehicle, it should already be at least in the upper half of its remaining rated capacity. The one amp charger should charge it within a day, perhaps two. ---
**Author:** ![echoreply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/echoreply/32/3641_2.png) [@echoreply](https://boards.straightdope.com/u/echoreply)\ **Post date:** [May 27, 2020, 9:39pm UTC](https://boards.straightdope.com/t/can-a-1-amp-trickle-charger-fully-charge-a-car-battery/854560/5 "2020-05-27T21:39:25Z")
Some trickle charges will not charge a battery unless the battery has at least some voltage in it. For example, the Battery Tender Jr. will not charge until it detects 3 volts. The $10 Harbor Freight battery tender requires the battery be at 12 volts.