How to get the most power out of the MPT-7210A boost charger

In normal solar power systems used to charge batteries, the solar panel voltage is higher than the battery voltage, and a traditional charge controller can then be used. Sometimes, you want to charge a battery that has a higher voltage than your panels, though, such as an ebike battery with a small solar panel. For that you need a boost charge controller or boost converter. Very few are available, and the most common seems to be the MPT-7210A "MPPT" charge controller. It can also be used to turn any DC power supply (even a free cast-off laptop charger) into a battery charger.

The MPT-7210 boost charge controller

I say MPPT in quotes because the MPT-7210A does not actually do Maximum Power Point Tracking. But it is still a valuable tool -- it is basically a programmable boost converter that has a setting for the minimum input voltage to use. That way, as it ramps up power and the voltage of your solar panels is pulled down, it does not just keep increasing current and collapse the input voltage to zero like a normal boost converter would do, but instead stops when the voltage is pulled down to your setpoint. See my previous post on DIY power stations for more discussion on how MPPT technology works.

How the MPT-7210A works

The MPT-7210A is described as a 10A/600W unit, but many of the reviews comment that they cannot actually get 10A or 600W out of it. For example, when supplied 12V, only a small amount of current (6.5A) is used to charge a battery at 54V, only providing 1.24A/67W of charging power. Where is the 600W?!

The answer lies in the internal limitations of the boost converter design. To get more current at the output, the input and output voltage need to be closer together. In the example of charging a 54V battery, a 48V input allows for 6.6A/360W of charging power. Much better! A 52V input would do even more power.

Charging a battery using a DC power supply

Testing the capabilities of the MPT-7210A

Mapping out the input/output voltage and current for a variety of scenarios gives the following results. I used a DC power supply for the input, always with more current available than the MPT-7210A would take. The 13s data was collected by charging one of my actual 13s (48V) packs. The rest was taken using a DC electronic load, the Multicomp Pro MP710259, instead of a battery.

Application	V_in [V]	I_in [A]	P_in [W]	V_out [V]	deltaV [V]	I_out [A]	P_out [W]	efficiency [-]
7s 25.9V nom 29.4V max	12	6.83	82	28	16	2.46	69	84.0%
	18	7.96	143	28	10	4.6	129	89.9%
	24	9.6	230	28	4	7.51	210	91.3%
10s 37V nom 42V max	12	6.58	79	40	28	1.66	66	84.1%
	18	5.92	107	40	22	2.37	95	89.0%
	24	5.82	140	40	16	3.18	127	91.1%
	31	7.16	222	40	9	5.17	207	93.2%
13s 48.1V nom 54.6V max	12	6.6	79	54	42	1.24	67	84.4%
	18	5.1	92	54	36	1.48	80	86.9%
	24	4.44	107	54	30	1.78	96	90.0%
	31	4.31	134	54	23	2.27	122	91.7%
	41	5.3	217	54	13	3.75	203	93.4%
	48	7.79	374	54	6	6.6	360	96.2%
20s 74V nom 84V max	12	6.24	75	80	68	0.75	60	80.1%
	18	4.41	79	80	62	0.83	66	83.6%
	24	3.6	86	80	56	0.92	74	85.2%
	31	3.12	97	80	49	1.06	85	87.7%
	41	2.87	118	80	39	1.32	106	89.7%
	48	2.92	140	80	32	1.6	128	91.3%
	60	3.56	214	80	20	2.51	201	94.0%
	67	4.69	314	80	13	3.74	299	95.2%

Test setup for determining the max input/output current for a given set of input/output voltages

So, the unit works, but this behavior is not very convenient if you want to charge a high voltage battery with a relatively low voltage source. The max power point voltage of a solar panel is typically in a pretty narrow range and not adjustable. But you can use the table and the following chart to estimate how much charging performance you can get. No matter how much solar or DC power is available, this is the max the unit will output.

MPT-7210A output current vs input-output voltage difference

High power charging is possible! Requires an input voltage very close to the output voltage.

Conclusion

The MPT-7210A boost charge controller works, but you have the understand the limitations to avoid getting frustrated with it. It can only charge with high power when the input voltage is close to, but still below, the output voltage.

That said, I'm still not totally sold on this thing. To avoid some of the trouble, you can also charge batteries with a standard bench power supply. For obscure solar setups the MPT-7210A is probably needed, but oddly there is no way to stop charging once current reaches some low threshold (indicating a fully-charged lithium ion battery). That's means you can't really let it charge unattended unless you are certain the battery won't ever fully charge. An odd device.

Possible alternative products: This no-name one (does not appear to accept a DC power supply input), and a Drok boost converter.

Have you used any of these units? What did it take to get it working well?

-Mike