How To Charge Your Batteries.

After you install a charge controller and battery monitor, they will not work unless you set them up properly. Proper setup varies from boat to boat, and is a bit arcane. For this reason it has been my experience that most people have their systems set up wrong and do not understand how they work. This leads to shortened battery life, confusion, and frustration. The goal of this page is to provide the minimal information to help someone with a similar system get it setup correctly. I also provide links where appropriate for those wanting more information. I welcome all feedback. Nothing on this page is original. I have just tried to collate all the great information I used into one place.

Note that all of this information is specific to my setup. Most of the information applies generally to similar setups, but a few of the specifics vary. My setup is:

  • 2 x Trojan T-105 Flooded Batteries
  • Midnite Solar KID Charge Controller
  • Victron BMV-600 Battery Monitor
  • 3 x 100w Renogy Flexible Panels

Battery Monitors

Amp hour counting battery monitors are the norm on boats. These are confusing and imperfect instruments, but far, far better than just an amp meter. It seems someone has finally invented a much improved battery monitor, but for those of us stuck with the amp hour counters, this is the minimum you must do to get them working.

  1. Install them properly
  2. Configure them properly
  3. Understand what a synchronization is and how to use it

Installing a battery monitor it not as straight forward as it seems, and many, many problems are due to installation errors. Verify your installation if you did not do it. All you need to know is here.

To configure your system, you need to know three pieces of information, all of which are specific to your batteries:

All of these pieces of information vary depending on the type of batteries you have and who manufactured them. Note that Amp Hours can be listed in a lot of ways. Make sure you have the 20 hour rate. Your best bet by far is to look up this information online or call your battery manufacturer. Here are the values I use for Trojan T-105 batteries:

  • Amp Hours – each pair of T-105 batteries provides 225 Amp Hours at 12 volts.
  • Peukert’s exponent: 1.24
  • Charge Efficiency Factor (CEF): 89%

If you get the Amp hours number wrong, your battery monitor will be useless. Peukert’s and CEF are less important, but still important enough to make it worth your time to look them up or ask around online. Read this for more details.

Once you have this information, just look at the owner’s manual for your change controller to determine how to configure it with these three values. On the Victron BMV, they are labeled as follows in the setup menu:

  • Cb: Battery capacity Ah
  • CEF: Charge Efficiency Factor
  • PC: Peukert exponent

Understanding synchronization is another area that leads to major confusion for many users. When you look at your battery monitor and it says the batteries are 90.5% charged you are inclined to believe it. Don’t do it. It is all a lie. You charger is just guessing what the state of your batteries is. Often it will make a terrible, terrible, guess. It is not unusual for a controller to say your batteries are 80% charged when they are in fact only 50% charged. This is the main problem with the Amp hour style counters. If you have your monitor installed correctly and setup correctly as above, understanding synch is the final step to avoiding this.

Your battery monitor works by watching how much power is taken out of your battery. As power is removed, it deducts that from the total Amp Hours you specified (modified using the Peukert’s exponent you specified). As power it put back in, it add that back to the total (modified using the CEF you specified). In this way it keeps a running total. The problem is that over time this total will slowly start to get inaccurate as the error in the estimates the controller is making start to build up over time. To counteract this, the controller periodically needs to be told “the battery is now full”, so that it can start its counting process over. A battery monitor will only be accurate if it has recently been told correctly that the battery is full. To guess when the battery is full, the default values the manufacturer plugs into your monitor are terrible. If you keep using them as a basis for charging, you will be chronically under charging your battery.

There are two ways you can fix this: 1) turn off all automatic synchs and only do manual synchs 2) change the default values to more realistic values.

Turn off all automatic synchs is the best option IMHO. Especially if you live on solar alone. You basically change the settings in the monitor to make sure it never synchs the counter. You then manually synch the counter when you know your batteries are full, or almost full. The Charge Controllers section below explains how to know when to do a manual synch.

Change the default values is not a good idea in my opinion, but if you cannot stand the notion of manual synchs, it is far, far better than leaving the defaults in. For the Victron BMV, the values you want to change are: Charged Voltage, Tail Current, and Charged Detection Time. Set these as follows:

  • Charged Voltage: change to the float voltage you specify in your charge controller, explained below. 13.2 for Trojan T-105 batteries.
  • Tail Current: Set to 1%
  • Charged Detection Time: if you refrigerator, or other large loads, cycle on and off, you need to set this to longer than they run for. For example, if you refrigerator turns on and runs for 10 minutes at a time, but never more than 10 minutes, set this to >10 minutes. Say 15 minutes. This is because things that draw a large load, like your refrigerator, can mess up how the controller determines tail current.

Charge Controllers

Most charge controllers are three stage controllers. The three stages are BulkAbsorption, and Float. Bulk is when the charger is throwing everything it can at the battery to charge it as fast as possible. This is where you get most of your results, but it can only be done for so long before the controller needs to slow down a bit. The controller decides when it needs to slow down based the battery voltage. This is called the Absorption Voltage. Basically the controller throws a bunch of amps at the battery and the battery voltage starts to rise. It will gradually rise until it hits the Absorption Voltage. At that point the controller will switch to absorption mode and start to put out less amps, but still at the Absorption Voltage. The amount of amps it puts out will decrease over time. Eventually the charger will switch to float mode. In float mode, it will drop the voltage from Absorption Voltage to Float Voltage.

The correct Absorption and Float Voltages need to be configured in your charge controller. Do not use the default settings. Look up the correct settings for your batteries online or call the battery manufacturer. For Trojan T-105 batteries, the correct settings are:

  • Absorption Voltage: 14.8
  • Float Voltage: 13.2

I have a 30 amp solar controller. My panels normally put out about 15 amps. So my charging looks like this:

  • In the morning the batteries will be low since we have been using them all night with no charging from the solar panels. Battery voltage will depend on how much power you used during the night, but lets say they are at 12.2.
  • One the sun hits the panels, the controller turns on and will go into Bulk mode. It will put out as many amps as the panels can make (up to 15), and watch the battery voltage.
  • The battery voltage will gradually rise as the batteries absorb amps from the controller. Eventually it will rise all the way to the Float Voltage I have configured: 14.8.
  • When this happens, the charge will switch to Absorption mode. In absorption mode it will put out less amps and the amps will taper off over time. It will put out these amp at the absorption voltage, so you will see the voltage stay at about 14.8, but the amps decrease suddenly when it switches from Bulk to Absorption, and then gradually during absorption.
  • Once Absorption mode is over, the controller will switch to Float mode. In float mode it will drop the voltage from the Absorption Voltage you have configured, to the Float Voltage you have configured. At this point your battery is mostly, but not fully, charged. The controller will have to stay in Float mode for quite a long period of time to fully charge your battery. It will stay in Float mode as long as the panels are putting out enough power for the controller to continue to operate.

You should learn to recognize just by glancing at your battery monitor what your controller is doing. You basically want to be looking at two screens: voltage and current. On the Victron BMV, voltage is labeled V and current is labeled I. If voltage is low and current is high, you are in Bulk mode. If voltage is high you are either at the very end of Bulk or in Absorption mode. If both voltage and current are low, you are in Float mode, or your controller is off.

By learning to recognize what mode your controller is in, you will know what your batteries are doing.

Controller synch Revisited

Now that we understand what our controller is doing, we can get a better idea when to do our controller synch. Assuming we have disabled automatic synchs, it will only synch when we manually trigger it. If you are using solar, synchs will always happen just before sunset. That is when the batteries are at their highest state of charge for the day. Ideally it will be a day where Bulk and Absorption completed early in the day and the controller has been in Float for most of the day. At the end of the day, ideally about an hour before sunset, do a manual synch of your controller. This is basically the closest to full your batteries will ever be assuming you live on your boat and only charge with solar. If you only use it for weekends or for vacations, then synch as soon as you arrive on a Friday night. The key is to figure out when your batteries are closest to 100% full in your weekly cycle and do the synch then. You batteries may not be truly 100% full, but this is as close as you will be able to get.

Note that if you charge controller never enters Float mode, or even worse if it never enters Absorption mode, you are not generating enough power. You need to either tweak your system to get more out of what you have, or you need to supplement your solar charging with more panels or another charging source.

A Few Advanced Settings

Once you have your system tuned and operating well, you may want to adjust a few more settings:

  • Absorption Duration: my controller decides when to switch from Absorption to Float mode based on a timer. If this duration is too high, you will be chronically overcharging, which is bad for your batteries. If this duration is too low, you might be chronically under charging, which is bad for your batteries. Try setting this duration to 30 minutes for every 100 Ah in your battery bank. If you notice you are having to add a lot of water to your batteries, shorten this duration. If you are having sulfation problems, increase this setting.
  • Low and High Voltage Alarms: these can warn you if something goes awry. A good starting point is to set the high alarm to .3 volts above your Absorption Voltage and the low to 12v.
  • State of Charge Alarm: Set this to 50%, so that if you batteries get low you are aware of it.
  • Even more: read the excellent “Programming A Battery Monitor” article from Compass Marine.

Sources for More Information

In addition to the links in the text above which give more details on specific topics, the below provide more information of a more general nature.


  • The manuals for your charge controller and battery monitor. Re-reading them every 6 months or so as you learn more is a huge help.
  • Battery University: in-depth information about all things battery.
  • Compass Marine How To Articles: detailed tutorials on how to do many electrical projects.
  • Great Discussion from s/v Luckness on batteries, solar, and charge controllers.
  • Trojan Batteries Users Guide: great if you have Trojan batteries.
  • Midnite Solar Forum: if you do not use a Midnite Solar controller, your controller manufacturer web site might also have forums.
  • Sailnet and Cruisers Forum and good places to ask for help and read tons of existing threads on all these topics.

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