How much current is drawn from the 12V (or 24V) battery when running an inverter connected to a battery?
Documented in this article is a common question relating to the inverter current draw and a breakdown of expectant voltage and efficiencies.
Question - How much current is drawn from the 12V (or 24V) battery when running an inverter connected to a battery?
Answer - The simple answer is- divide the load watts by 10 (20). E.g. For a load of 300 Watts, the current drawn from the battery would be:
300 ÷ 10 = 30 Amps (300 ÷ 20 = 15 Amps)
Notes - It is the actual load watts, not the inverter rating that counts. Therefore, a 1500W inverter with a 500 Watt load would be 50 (25) Amps, not 150 (75) Amps. The same inverter with a 1200 Watt load would draw 120 (60) Amps.
How long will my battery last with an inverter load of 1000 Watts?
For a more accurate calculation of battery current: Divide load watts by actual battery voltage, this will be in the range 12-14V (24-28V).
Then to allow for inverter efficiency, typically 85%, divide the figure by 0.85. Thus:
For a 300W load at 12V....300 ÷ 12 ÷ 0.85 = 29.4 Amps.
For a 300W load at 14V....300 ÷ 14 ÷ 0.85 = 25.2 Amps.
You can see the simple divide by 10 gives an easy "worst case" guide.
For a 300W load at 24V....300 ÷ 24 ÷ 0.85 = 14.7 Amps.
For a 300W load at 28V....300 ÷ 28 ÷ 0.85 = 12.6 Amps.
You can see the simple divide by 20 gives an easy "worst case" guide.
Note: Figures in brackets are for 24V systems.
Question - Can the charging management system contained inside AMPTRON batteries cope with pulse charging that is typically utilized in smart chargers and solar chargers?
Response - The answer to this interesting question is not a simple yes/no response. The simple response is yes, the batteries can "cope" with pulse charging in the short/medium term, however if a pulse charger is used constantly then there is a longer term impact. Let me elaborate a bit more.
One of the issues with a pulse charger as opposed to a constant voltage/constant current charger, is that the voltage peaks of the pulses are not that well controlled, and are often above the maximum allowed voltage. If a constant voltage charger exceeds the maximum voltage of the batteries, then the internal Battery Management System will simply disconnect and with virtually no longer term impact. However, if a pulse charger regularly exceeded the maximum voltage, then the internal BMS will be constantly disconnecting/reconnecting the cells, and besides that, the internal MOSFET switches can't always react that quickly so for small fractions of seconds, the cells will be exposed to over voltage situations.
As a result of the above, both the cells and the Battery Management System over time will start to suffer some damage/excessive wear that will affect the overall battery life. Now it is very difficult to estimate the magnitude of this impact in terms of a % of the total battery life, since there are many factors that will determine the overall impact.
Further, any lead acid charger (not just the pulse chargers), typically won't be able to charge the batteries to 100% State of Charger, because the constant current phase tends to end too soon. This usually only affects the top few % of the state of charge, so it is not a major impact to the battery.
Another potential issue, is when the charger, after reaching a full charge (or at least when the charger determines or thinks the battery is full) and having switched over into the float charge mode, sometimes this will return to the bulk/boost charge phase again. This switch over sequence is usually based on voltage, however some lead acid type chargers have this voltage threshold too low for Lithium batteries. For AMPTRON LiFePO4 batteries, the charger should return to bulk/boost charging around 13.2V, but we have seen lead acid chargers only kicking in around 12.8V which will mean the battery is already down below 30% before the charger properly starts charging again.
So the bottom line is a pulse charger can be used if intended for short/medium term usage, however it is not recommended for long terms usage due to the issues described above.
Question - Can Amptron LiFePO4 batteries charge from a standard commercial solar module or does it require a special module? And is there a need for a controller?
Response - The battery can be charged from standard commercial solar modules, but it definitely requires a solar charge controller. Now, when it comes to solar charge controllers, our batteries can be charged from most nominal 12V controllers however there are compromises. When it comes to modern solar controllers, there are 2 main characteristics that needs to be considered:
To sum all of this up, yes you can charge our Amptron batteries with standard commercial solar panels, but a charge controller will be needed. As for charge controllers, an MPPT charge controller will definitely be the preferred option unless lowering cost is really necessary, and as for MPPT charge controllers, it is preferable to use an MPPT charge controller that can charge the battery with an LiFePO4 profile.
Question - Can I install this under the bonnet in a car engine bay?
Response - As for installing the battery under the bonnet, the main concern is heat. Our batteries are designed to operate up to 65 degrees C. This would be ok under most conditions, however there may be situations where the ambient temperature gets very hot such as in hot summer conditions and when the engine is working particularly hard, where the temperatures under the bonnet could cause the battery temperature to exceed these tolerances.
Therefore there is no simple answer to this. It would depend significantly on where the second battery tray would be located in your engine bay (i.e. it would make a significant difference if it was close to the front near the radiator with good airflow, vs. tucked away behind the motor at the back), and the conditions that you would run your vehicle under.
Our battery management system does contain a thermal valve that would disconnect the battery should it overheat to protect the battery from excessive damage. However, if the battery is running hot much of its life, it would shorten the battery life.
Therefore, as a basic rule-of-thumb we do not recommend it. However, we do have several customers who do install our batteries in the engine bay but they understand the potential impact. To date, we have not yet had any reported issues. However, if there is evidence that the battery has been operated under excessively hot conditions, then that may void the warranty.
Question - Do I need to upgrade my charger to a LiFePO4 charger for a 12V 200Ah / 100A Continuous discharge Lithium LiFePO4 Battery. I currently have a redarc BMS30 installed?
Response - Our battery management system allows our batteries to be charged with the older BMS30 (that does not have a Lithium profile), however there are a few things you should note: