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Battery Management Guide

Understanding your batteries is an important part of running an off grid system.  Regular maintenance of your batteries will ensure they work optimally and keep in service for as long as possible.

Flooded Batteries

Flooded batteries require regular fluid level checks.  You should check these ideally every month, but at least every 3-6 months, depending on the use of your system.  The fluid should cover the plates.  To top up your batteries use de-ionised or distilled water, you can get this from any motor factors or battery shop.   Some batteries will have a high and low mark.  If you have a large bank of batteries you can get watering systems that either allow you to fill them all from a single point, or auto watering systems that you just fill a tank.  We can provide watering systems for most batteries that we supply.  If the fluid level is low then the battery will not hold its charge and the plates might get warped causing irreparable damage to the battery.

Battery charge voltages

Each battery type and each manufacture have their own recommended battery charge voltages.  Most charge controllers come with a generic battery type options (Sealed/Flooded/Gel).  You can select your battery type and it will give you the most likely settings for your batteries.  It is important to charge your batteries at the correct voltage.  Some manufactures have slightly different voltages than the standard, for example Trojan batteries have a slightly higher charge voltage than standard lead acid batteries.  You should check with the manufacturer or the spec sheets for the recommended charge voltages for your batteries.  You can set your MPPT charge controller to the battery type or set the specific custom batteries voltages you require.  Our Tracer BN series and the Outback MPPTs can both be set to specific voltages.  Normally you will see an Equalise, Boost and Float voltage to be set. If you are using NiFE batteries these require a higher than normal voltage, we list the voltages on our NiFE page here.

Battery Equalisation

It is important to equalise your batteries regularly.  Equalisation provides a higher charge voltage to the batteries and ensures that all the cells have a balanced equal charge.  If your batteries are not holding enough charge equalisation can help rectify this and bring the battery back to hold more charge.  Generally you should run a 2hr Equalisation charge once a month. You will need a good sunny day to run your equalisation if running from your solar.

The tracer MPPT does a 2 hour equalisation charge at 9am on the 28th of each month.  If you need to manually do an equalisation charge on a good sunny day set the date to the 28th and time to 07:58 to let it do the equalisation.  Or on the night before set the date to 27th so the following morning it will equalise.  It might be necessary to do this more than once if the battery is in need of a good equalisation. 

Outback MPPT you can set an auto equalisation to happen after a customised amount of days.  You can also set an equalisation cycle to happen at any point from the menu on the MPPT. To set Equalisation from the Mate, you can watch this video here.

If your batteries are not holding their charge, then equalisation can help bring them back. 

Recovering Flooded Batteries

You can give a higher voltage to flooded batteries to get them to start taking charge and then bring the voltage back down to the correct level once they are taking the amps.  A simple way of doing this is to increase the equalisation voltage slightly to see if this helps.  We would advise for a flooded lead acid battery making it 15V, and run for a 30min - 1hr charge.  Then reduce the equalisation voltage back down to the recommended level and run a 2hr - 4hr equalisation charge.

You can bring flooded batteries back by using even higher voltages but Extreme Caution should be taken if you plan to put a higher voltage into the batteries, and you should monitor the volts and amps carefully.  Never attempt to put a higher voltage into any sealed battery. Never put a higher voltage into a battery unless you are 100% confident in what you are doing and able to monitor both the volts and amps you are working with.

Always ensure that you have topped up the water levels before trying to equalise or recover a battery.

Low batteries

It is common in winter to find that your batteries are low, the power you are generating is much less in winter but your usage is usually more than in summer.  If your batteries are low it can be hard for them to get back up to charge in your normal cycle.  You are best to give them charge either off the mains or generator, or by running your system from another battery for a while allowing all the charge to go into your other battery until it is fully charged again.

Your battery needs to get up to 12.7v and stay there for a sustained period to allow the battery to absorb the charge, known as floatation. If your battery is not reaching floatation for a sustained period, it will struggle to hold a charge.

If your batteries are low and not holding charge then you will need to run an Equalisation charge on them to recover the power of the battery. Please see above on equalisation

If your batteries are running to this point you should look at your system to see if you need to add more solar panels or other charge to the system to support your usage as you are using more than you are generating.

State of Charge

The below table shows the resting voltages of the various state of charge for most batteries.

% Charge 12V 24V 48V
100% 12.6 25.2 50.4
90% 12.5 25 50
80% 12.42 24.84 49.68
70% 12.32 24.64 49.28
60% 12.2 24.4 48.8
50% 12.06 24.12 48.24
40% 11.9 23.8 47.6
30% 11.75 23.5 47
20% 11.58 23.16 46.32
10% 11.31 22.62 45.24
0% 10.5 21 42

The resting voltage is often hard to determine exactly as when charge is being given a higher voltage is showing and when power being taken a lower voltage shows.  If you turn on a high draw appliance then the battery voltage will instantly drop as you draw power for that item, this isn’t the % of discharge, this is while the power is being used.

% Charge readings on controllers

You can pretty much ignore the % SOC reading on the meter, its useless and shouldn't be on there, they usually base a full 100% battery on 14.4V, so as soon as you stop charging the battery then it drops to around 60%, so when the sun is out its showing 100% and as soon as it goes down it drops to 50-60%.  Please use the table of voltages above as a guide rather than the % state of charge.

Accurate Battery Charge via Hydrometer

The best way to see the state of charge of a flooded battery is using a Hydrometer, this is far more accurate than relying on the voltage, and will show if there is a problem with any individual cell.

There is a good guide to checking your specific gravity from Rolls Batteries which you can read here.

The following shows the approximate state of charge at various specific gravities at 77ºF / 25ºC.

Charged Specific Gravity
100% 1.255-1.275
75% 1.215-1.235
50% 1.180-1.200
25% 1.155-1.165
0% 1.110-1.130

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