What happens if a charger only has access to limited current?

I have some solar cells I want to use to charge a large battery bank. There are 4 solar panels, each of which is 24VDC nominal. The battery bank floats around 106VDC. I was unable to find a DC-DC converter that is high power enough to convert the output of the panels to the voltage appropriate to charge the batteries (115VDC).

Instead, I want to use the AC-DC charger that came with the batteries coupled with a DC-AC inverter. My plan is for the solar DC output 24VDC to get converted to 110VAC which will then power the battery charger. My only concern is what the charger will do if its current supply is limited. The solar panels will probably only output 3-5A at 110VAC. The charger is rated for 15A.

What will happen if the charger is not able to pull as much current as it wants from the solar panels?

There are various different things to say about what you wrote. Unfortunately this forum software doesn't provide a easy way of quoting your text and replying to the individual points. Are you listening, Cody, Joe, and others?

First, you can't put watts into the batteries that aren't coming out of the solar cells. If there is less coming in than what the batteries want, then something has to give. How to deal with that depends on the batteries. Some types don't mind long term trickle charge while others do. Before you decide how to do any conversion, you have to know what you want to convert to. You said the batteries need 115V to charge, but that must only be for part of the charging process. I seriously doubt your batteries want to be charged with anything as simple as a 115V fixed voltage source. Most of the charging specs are probably current as a function of voltage, time, and maybe temperature.

First converting the solar panel output to AC, then converting that back to DC to charge the batteries makes no sense to me. At best that will be inefficient. Is getting a off the shelf solution important for some reason? Why not make your own boost converter from 24V to the battery voltage. Use a microcontroller so that it can provide the proper voltage and current over time and temperature that your batteries require. It can also simply shut off when there isn't enough power to charge the batteries properly. You could even store low power over time from the solar cells in a large capacitor, then run the batteries at their desired charge voltage and current for a few 100ms at a time - if this is OK with the batteries.

Also, you have 4 panels that apparently provide 24V each in sunlight. If you can put these in series, that gives you 96V max at full sun, which will probably be easier to boost to 115V than 24V. The input and output voltages will be a lot closer and it only has to handle 1/4 the input current.

Thanks for the reply. I'll try to address all of your points one at a time.

I don't understand what you mean about not being able to put watts into the batteries that the panels cannot provide. The LiFeMnPO4 batteries I have are able to be charged at anything up to 300A continuously. If there are only 3A available for charging, the batteries are not going to scoff at me, they'll just take 33 hours to charge completely. The 120VAC to 115VDC charger that I have can only charge them at 10A max.

The 120VAC to 115VDC charger I have has 3 modes: 10A fast charge for bringing the battery cells up to ~90% quickly, then 5A slow charge, then finally 3A sawtooth that charges for 5 minutes, then shuts off for 5 minutes to allow the cells to settle and then repeats for a few cycles until the batteries are fully charged. Here's the specs for it: http://elitepowersolutions.com/products/product_info.php?cPath=9&products_id=139

An off the shelf solution is best because it is faster and cheaper. If there's no need to reinvent the wheel, then I'm not going to. I already have the 120VAC charger which is 95% efficient. The 24VDC to 120VAC inverter I'm looking at is >90% efficient and it's only $90: http://www.powerbright.com/ML900-24.html DC to DC converters powerful enough to handle the amount of power that will be coming off the panels cost upwards of $800 (although I was never able to find one that could do exactly what I needed).

The reason I want to put all of the panels in parallel instead of series is because 24VDC to 120VAC inverters are cheaper and easier to find that 48 or 96VDC inverters.

Thanks again for your comments however my main question still remains unanswered: what will my charger do if it can't pull as many amps as it wants to pull?

Hi Olin,

We are listening... I have a solution in mind. We actually have a small list of features we would like to add to the forum.. Maybe it is time to start on that list.

If this is a one-off, then it will be cheaper to find something off the shelf than designing your own. However, you are going a long way around to find a solution when all you really need is a boost converter with some current control and voltage readback.

As for what your inverter will do when the charger tries to draw more power than it can supply, or the charger will do when the inverter can't supply the specified voltage, there is no way to know unless you can find these behaviors specified in the datasheets. This is one of the drawbacks of using off the shelf black boxes.

What is your expected volume? Your solution sounds very expensive compared to a DC-DC boost converter designed for the purpose. That should be a few 10s of $$ in parts at worst, but of course some up front design time. It won't take a lot of units to pay that back at $100s savings per unit.