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

For flooded batteries always ensure you have topped up the fluid levels (see above) before 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 08:58 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.  Alternatively you can set the Boost voltage to the equalisation voltage and set the number of hours you want to equalise for and the boost mode will do the EQ for you. Remember to re-set the voltage back down again afterwards.

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. 

You may well need more than 1 equalisation, if you say have 400Ah of batteries and a 40A MPPT it will take 400AH/40A = 10Hrs of the charger working at full charge to get the capacity to the batteries, so you would need to set a tracer to 5 EQ chargers before you were near capacity again, and most likely you will be getting less than 40A so you will need more to account for that. Simply doing 1 x 2hr EQ charge if the batteries are completely flat will not be enough.

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.

We have a charger for giving constant voltage charge which you can use for recovery here

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.

Low NiFE batteries

If your NiFE batteries are low and not holding charge then you need to give them a good equalisation charge with upto 1.75V per cell for 24 Hours (17.5V for 12v, 35v for 24v). You can start with 1.7V per cell charge for 8-12 hours to see if that brings enough power back, and if they are still not holding charge then do the 1.75V for 24Hours. You indeally need to do this as 1 continuous charge, but if you can't as you are only using solar to EQ then discounnect the inverter to ensure that no power is used from the batteries in the evenings. Some equipment will not go to 1.75V per cell so work with 1.7V per cell if that is the case, or disconnect the equipment that doesn't like the higher voltage. For Example with outback equipment the FM80 will allow you to EQ at 1.75V per cell but the VFX see that as over voltage, so disconnect the inverter if using the FM80 for EQ.

If the above EQ charge does not recover the power from the batteries then you will need to do 3-5 cycle charges where you charge up to the 1.7v-1.75v per cell for 1-2 hours and then discharge the bank down to 1V per cell and then repeat the charge and cycle 3 to 5 times.

We have a charger for giving constant voltage equalisation charge for NiFE batteries 12V & 24V here and for 48V here

It is not possible to damage NiFE batteries from over charging, so if your charger is not able to be set as high as 1.75V per cell, you can connect your solar panels in parallel directly to the battery, without a charge controller, to give it a high voltage charge. Ensure you regularly check the fluid levels as the higher voltage will bubble off the fluid faster than usual. For a 48V bank you would connect panels in pairs to get a high enough voltage, and for 12v and 24v connect them in parallel.

New NiFE batteries require a few cycles before they obtain their full power, and if you find they have lost capacity then they require a few cycles to get the power back again. Give them a 1.75V per cell EQ charge as above, and then discharge them to 1V per cell and then back up again, do this 3-5 times.

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. We sell an acurate Hydrometer here.

There is a good guide to checking your specific gravity 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

Problems with a large battery bank?

Battery Bank cell tests for 2v or 1.2V cells

If you have a large battery bank made up of 2V or 1.2V cells you can see if one cell is causing the a problem and dragging down the whole bank. Simply test the voltage of each cell and see if there are any that are particularly lower than others. Ideally they should all read the same voltage. If you find one that is lower, then do a load test on the battery bank and observe the behaviour of that cell. If it drops rapidly or behaves erratically then there is probably an issue with that cell.

NiFe trick to keep running even if you find a damaged cell

One of the advantages of a NiFe bank is then you can actually just remove the damaged cell from the bank until a replacement can be added. This is a huge advantage for remote areas where a replacement could take some time to get, enableing the system to continue in operation. Simply remove the cell and then recalculate the voltages for the bank based on the remaining number, and re-program your controllers and chargers for the new voltages. For example:

  1.2V cell 24V
20 cells
19 cells
Equalise 1.65v 33v 31.35v
Boost 1.55v 31v 29.45v
Float 1.45v 29v 27.55v

Battery Banks made up from 6V & 12V batteries

If you are using a bank of 6V or 12V batteries then you can do the same test as above, if they are just wired together without bus bars you will observe that the batteries at the edge of your bank will drop faster in the load test. These batteries generally are working harder than the others in your bank as all the power is going though them. This is why you should use bus bars to connect the battery bank up so that the load is spread across the bank. Usually you will find that the batteries at the edge are the ones that fail first. If you do find this then you would be wise to re-wire with bus bars to extend the life of the bank.

If you find a failed battery you can remove the part of the bank that is causing the issue until a replacement can be found. Ie if you have a 24V bank made up of 6V batteries you would have to remove 1 set of 4 x 6V batteries including the damaged one to retain the bank as a 24V bank still.