I'm a newbie - I know V=IR, and VI=P (after considering power-factor in practical circuits), and that's about it. I need to get something built, and I'd like comment on whether my reasoning here is 'on track' or not. I'm not asking for help with designing a circuit (yet) ... I'm more trying to understand what that circuit needs to do, and why. (And whether I need to build one, or could buy one). Sorry it is long.
I have a portable load I wish to power, from a battery if no other power source is available. If another power source is available, it should power the load and charge the battery. One arrangement of charger/battery/load is that the battery is 'attached' to the load while being charged (as opposed to being contained 'contained' or 'detached' when charged).
Something like this:
(I don't know if this 'attached' arrangement is the right way to skin the cat. Possibly it is not. Mobile phones have roughly the behaviour I am wanting, and I hear that they use a 'contained' arrangement ... either the battery is connected to the charger (charging, while the external power-source powers the load), or the battery is connected to the load (discharging)... the battery is never connected to both the load and the power source at the same time. Some kind of fancy switch chooses whether the battery or the external power supply is powering the load, without interrupting supply.)
But plunging ahead with this 'attached' arrangement:
The battery a FusionAGM V-LFP-12-10 (PDF spec) is LiFePO4 ... but has an on-board battery-management-system such its charging characteristic is allegedly like a Sealed Lead Acid battery. Its features are:
In any case, the battery expects a 'CC/CV charging algorithm'. I understand CC/CV thus:
CC phase = 0.2C = 0.2 / 10Ah = 2A.
CC->CV trigger = voltage reaches 14.6V (being the 'cycle charge voltage' per the battery specs)
CV phase = to maintain 14.6V, keep dropping current as required
CV->Done trigger = current falls below 1.0A ('minimum charge current' per the battery specs)
Done phase = drop constant voltage to 13.8V (being the 'float charge voltage' per the battery specs ... 0.5V above the nominal voltage of the battery)
However, in my 'attached' scenario, the charger is NOT connected only to a battery - it is also connected to the load. This means that the charger:
Therefore, an important input into the behaviour of the 'charger' (for its CC phase, and to trigger the CV>Done transition) is a measurement of the current used by the load. And the charger is therefore at least a calculator.
And I can't buy one 'off the shelf' that does this (?). So I have to ... build a charger circuit of my own.
Or have I missed something? Thanks for helping me think this through.
I have a portable load I wish to power, from a battery if no other power source is available. If another power source is available, it should power the load and charge the battery. One arrangement of charger/battery/load is that the battery is 'attached' to the load while being charged (as opposed to being contained 'contained' or 'detached' when charged).
Something like this:
(I don't know if this 'attached' arrangement is the right way to skin the cat. Possibly it is not. Mobile phones have roughly the behaviour I am wanting, and I hear that they use a 'contained' arrangement ... either the battery is connected to the charger (charging, while the external power-source powers the load), or the battery is connected to the load (discharging)... the battery is never connected to both the load and the power source at the same time. Some kind of fancy switch chooses whether the battery or the external power supply is powering the load, without interrupting supply.)
But plunging ahead with this 'attached' arrangement:
The battery a FusionAGM V-LFP-12-10 (PDF spec) is LiFePO4 ... but has an on-board battery-management-system such its charging characteristic is allegedly like a Sealed Lead Acid battery. Its features are:
- "Built in State of the Art BMS controls cell balancing, over and under voltage protection and short circuit protection"
- "Charge using a regular lead acid charger with float mode that does not have auto rejuvination or auto pulse"
In any case, the battery expects a 'CC/CV charging algorithm'. I understand CC/CV thus:
- The charger delivers a pre-set 'Constant Current' (eg. 1/5 of battery Capacity in Ah .. written as 0.2C) letting the voltage of the circuit be determined by whatever the battery can drive, until that voltage 'gets high enough'
- The charger then changes modes: now it delivers a 'Constant Voltage'. As time progresses in this mode, it becomes less and less possible to jam coulombs into the battery, and so the current flowing into the battery at some point becomes so minimal, that it is time to stop.
- The charger decides to 'stop' when the current demanded by the battery falls below a certain limit. It now gives a (lower) Constant Voltage called a 'float' which keeps the coulombs already in the battery jammed in there (but is not really so different from the voltage the battery itself such that there is any real effort to jam more coulombs in).
CC phase = 0.2C = 0.2 / 10Ah = 2A.
CC->CV trigger = voltage reaches 14.6V (being the 'cycle charge voltage' per the battery specs)
CV phase = to maintain 14.6V, keep dropping current as required
CV->Done trigger = current falls below 1.0A ('minimum charge current' per the battery specs)
Done phase = drop constant voltage to 13.8V (being the 'float charge voltage' per the battery specs ... 0.5V above the nominal voltage of the battery)
However, in my 'attached' scenario, the charger is NOT connected only to a battery - it is also connected to the load. This means that the charger:
- In the CC phase, will have to provide additional Current for the load. But how much?
- In the CV->Done transition, will have to deduct the Current that the load is using (but how much?) from the 'battery now only taking <1A of current' decision, or it will never transition!
Therefore, an important input into the behaviour of the 'charger' (for its CC phase, and to trigger the CV>Done transition) is a measurement of the current used by the load. And the charger is therefore at least a calculator.
And I can't buy one 'off the shelf' that does this (?). So I have to ... build a charger circuit of my own.
Or have I missed something? Thanks for helping me think this through.