Solar Lead Acid Charge Controller

We tested a dozen lead-acid charge controllers and we liked this one for its simplicity, size, and waterproofing. To make it even easier to get your project started, we added a female 3.5x1.1mm connector to link up with any of our solar panels. Works with both 6V and 12V lead acid batteries.


This charge controller is perfect for charging 6V and 12V sealed lead acid batteries (SLA) from Voltaic solar panels. It utilizes a 3-stage charge system of bulk, absorption, and float, which incorporates overcharge protection. In addition, it has load output wires with built-in over-discharge protection and auto-restart.

Set-up / Pairing
To use this charge controller you’ll need a 6V or 12V sealed lead acid battery. You should not use an AGM or gel battery with this controller since it does not limit current and the preset charge voltages in the charge controller are not ideal for those 2 types. In addition, you will need a properly sized solar panel. It needs to have a operating voltage greater than the voltage of the battery, preferably 1.5x greater at least. For example, you can pair our 17 Watt 18V panel with a 12V lead acid battery.

An important thing to keep in mind is the max charge current of the battery. In-house we have a UB645k 6V SLA which has a max charge current of 1.35A, as well as a ExpertPower EXP1270 which has a max charge current of 2.1A. Because this controller cannot know what that limit is, it’s important to make sure your solar panel will not supply greater current than this amount.

Once you have these two items, connect the battery to the controller first. Then connect the solar panel. If you have a load, connect that last.


Voltage at Battery Terminals    6V Battery (V)    12V Battery (V)   
Absorption voltage 7.16 4.13
Float voltage 6.98 13.98
Cut-off voltage 5.7 11.4
Load restart voltage 6.34 12.68

The controller will adjust the charge stage based on the voltage of the battery.

  • In the first stage, it will pull down the voltage of the panel to just slightly above the battery voltage and funnel as much current as it can (constant current)
  • The second stage holds the voltage steady (at 14.3V for a 12V battery and at 7.16V for a 6V) while allowing current to slowly taper off
  • At the third stage, the controller will keep the batteries topped off at float voltage (13.8V for 12V battery, 6.98V for 6V battery)

For the last two stages, the controller regulates the voltage down to what it needs but passes through all the current. As a result, if the solar panel voltage is very high, then there is a low efficiency, which is why it is good to match your solar panel MPP voltage to the battery voltage.

Load Regulation For those users who are using the load wires, there are a couple voltages to pay attention to. The load wires are unregulated from the battery, and they continue to supply power until the battery voltage falls below 11.4V for a 12V battery, or 5.7V for a 6V battery. Keep in mind that this voltage is with a load applied. At this point the controller will disconnect the battery from the load.

It only begins to supplying power again when the battery voltage reaches 12.68V for a 12V battery and 6.34V for a 6V battery. The controller will automatically connect the battery to the load leads when this happens, with no need for user interaction and making it suitable for a long-term solar application.

Pass-through Furthermore, the controller allows pass-through charging. What this means:

  • If the load is less than the solar panel input, the power in will be divided between the load and charging the battery
  • If the load is equal to the solar panel input, the power in will pass directly to the load, and no current will go in or out of the battery
  • If the load is greater than the solar panel input, the power in will pass directly to the load, and the load will pull current from the battery to supplement it
Lead Acid Battery Voltage: 6V & 12V Maximum Charging Current: 5A Maximum Load Output: 5A Maximum Voltage: 24V
Use with deep cycle lead acid batteries. Not for use with Gel or AGM.

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