Lithium LIFEPO4 Battery - Prevent more than 50% Discharge

Thread Starter

v2birit

Joined Mar 5, 2019
17
Hello,
I have a 30AH LFP (LIFEPO4) battery, which is being charged by a Solar Charge Controller.
Need to Disconnect the load if the Battery falls below 50% State of Charge. (to extend Battery Life)

The Solar Charge Controller, that i use currently, does have a LVD(Low voltage disconnect), but that is voltage based & since LFP battery has a very flat charging/Discharging curve, that voltage based disconnect is not very reliable.

So does anyone have a circuit or a link to a Solar Charge Controller that Disconnects the load when the battery actually falls to 50% charge.

Thanks.
 

Rich2

Joined Mar 3, 2014
172
The 50% rule is usually for lead acid batteries. I think lithium can be safely discharged much lower without damage, something like 95%.
 

Audioguru again

Joined Oct 21, 2019
1,779
The 50% rule is usually for lead acid batteries. I think lithium can be safely discharged much lower without damage, something like 95%.
Please do not guess (wrongly). Look it up.
A Lithium rechargeable battery is destroyed if its voltage gets less than half the fully charged voltage.
It should be disconnected when its voltage is 75% of its fully charged voltage.
It is stored when its voltage is 88% of its fully charged voltage.

A Ni-MH battery can be discharged to zero with no problems.
 

Rich2

Joined Mar 3, 2014
172
Please do not guess (wrongly). Look it up.
A Lithium rechargeable battery is destroyed if its voltage gets less than half the fully charged voltage.
It should be disconnected when its voltage is 75% of its fully charged voltage.
It is stored when its voltage is 88% of its fully charged voltage.

A Ni-MH battery can be discharged to zero with no problems.
I was talking of capacity not voltage.
 

Audioguru again

Joined Oct 21, 2019
1,779
It is easy to measure voltage but measuring capacity is complicated. I think most or all products have a charge controller that disconnects the battery when its voltage becomes low as I said.
To measure capacity you must discharge it to a recommended low voltage then measure how many mAh it takes to fully charge it.

Of course the capacity is higher if you charge it to its maximum allowed voltage then discharge it to its minimum allowed voltage but then its life is shortened.
 

oz93666

Joined Sep 7, 2010
737
I was talking of capacity not voltage.
I have to agree with Rich .... what we are trying to get from a battery is maximum KwHrs of storage over useful life ....

What I mean by this is , suppose you only use 50% Capacity , but double your cycle life ... you have gained nothing .... In fact you've lost , because you have to have a battery twice the size ....

In lead acid there is definitely a gain ... and in li-ion there is a big gain from keeping top voltage below 4V .... But with LiFePo4 there is no significant gain ... just a fairly small increase in total power stored over lifetime , which does not outweigh the need to have a battery twice as large (needed if only discharging 50%)..
There is a misinformation on the web , people have got used to limiting their DOD with other battery chemistries , and apply it to LiFePO4 when it is inappropriate...
I run these batteries and use the full 100%

The chart below is for LiFePO4


In solar applications the battery will very rarely be fully discharged , this will only happen after a long period of very cloudy days ...Most times over 24hrs less than half capacity is used , then the the battery is charged up again to 100% .... so in practice you will average about 50% DOD anyway ... giving you 6,000 cycles (18 Years) ... and even after these 6,000 cycles your battery still works it's just that it's capacity has dropped to 80% it's original storage capacity!!

So the best plan is Just to keep it fully topped up and use as required .
 
Last edited:

Audioguru again

Joined Oct 21, 2019
1,779
It will be complicated to measure the DOD (Depth of Discharge) since it is measured in the amount of its rated mAh energy (capacity) that has been used. The chart above shows that you gain many numbers of cycles when you use a low amount of DOD.
 

oz93666

Joined Sep 7, 2010
737
It will be complicated to measure the DOD (Depth of Discharge) since it is measured in the amount of its rated mAh energy (capacity) that has been used. The chart above shows that you gain many numbers of cycles when you use a low amount of DOD.
The chart shows that with 50%DOD we get 6,000 instead of 2,000 .... this may sound impressive but energy stored over life is only increased from 2,000 units to 3,000 units ...a 50% gain and will not warrant having a battery twice the size which will be needed ...

Lead acid and Li-ion show a much much bigger gain by limiting DOD.

Li-ion cycle life increases from 400 to 3,000 with 65% DOD(limiting top voltage) around 500% gain ... ten times more.!
 
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