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Firstly, I am aware that a similar question exists.

That being said, I have built a portable power station comprised of a 24V 25Ah LiFePO4 battery in an 8S1P configuration. From this I am able to run my DC loads and 500W AC inverter easily. Connected to the battery is a Victron SmartSolar MPPT 75/15 charge controller. The entire things works great, but I would like to add the ability to charge the battery via AC.

At this moment, dedicated 29.2V LiFePO4 battery AC chargers cost an arm and a leg! So I'm trying to get by with existing components that I currently have. Here is what I have in mind with a diagram to support ... I have a 12V SMPS with a rated current output of 30A, and intend to connect this to the PV input of the Victron MPPT. Now I have already read in many places that this is ill-advised unless the SMPS is current limited, else the charge controller could end damaging the SMPS. So I thought that I could make use of a boost converter that allows for constant voltage and constant current regulation. The one that I have lying around is (which is oddly the same in the Stackexchange question I linked to above):

enter image description here

with the following specs:

  • Module Type: DC-DC Boost / Step-Up Converter (Non-isolated)
  • Input Voltage Range: 10 – 60VDC
  • Output Voltage Range: 12 – 80VDC
  • Operating Current: 15mA (Measured at minimum output of 12VDC – Will increase with higher output voltages)
  • Max Output Current: 20A (If greater than 15A, additional cooling is required)
  • Max Output Power: 1200W
  • Constant Current Range: 0.5A to 20A (±0.3A)
  • Undervoltage Protection: Adjustable 9 – 50VDC
  • Switching Frequency: 150kHz
  • Conversion Efficiency: 92% – 97%
  • Other Protection: Over-Current Protection (25A Shutdown) / 30A Input Fuse (2x 15A Fuses in parallel) / Reverse Polarity Protection
  • Potentiometer Functions: Undervoltage Adjust Pot / Constant Current Adjust Pot / Constant Voltage Adjust Pot.

This diagram shows what I intend to do:

enter image description here

So my aim would be to set the voltage on the boost converter to perhaps 48V, but I realise I would then only be able to set the CC to 7.5A max.

After this mini-thesis, I would like to know from this community if this solution is viable?

DISCLAIMER: I do not have a formal electrical background - I'm a DIY'er.

Shalan
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    For the panels $V_{OC}\times I_{SC} < \text{ rated power}$. Chained conversions multiply (in)efficiencies. How do you intend to control the SMPS? How do you arrive at 7.5 A? – greybeard Jun 10 '23 at 04:29
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    A picture helps, but... Please give links to datasheets for each part of your setup and/or question. Help us to help you. (From a fellow dev, garbage in, garbage out, as you well know). Help us to help you. All 6 would be best. Panels, boost, smps, switch, mppt, LiFePO4, and anything else we are missing... – MicroservicesOnDDD Jun 10 '23 at 04:40
  • (Searching SmartSolar MPPT 75/15 charge controller specs for mentions of Li batteries, I found one (many of lead-acid).) – greybeard Jun 10 '23 at 04:53
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    Connecting a CC/CV to an MPPT seems reasonable to me. If the MPPT is able to take all the power, it should settle at both the CV and CC at the same time, as this is the maximum power point. You have one more converter in this diagram than you actually need, but that's reasonable since it's made from parts lying around in your junk box and you didn't have the exact converter module with both 240VAC input and CC/CV output. – user253751 Jun 10 '23 at 11:46
  • @greybeard: Apologies, the panels are actually 24.50 VOC and 5.16 ISC. – Shalan Jun 10 '23 at 12:28
  • @greybeard: How I arrived at 7.5A: SMPS --> 12V * 30A = 360W .... so after converting to higher voltage, the current will drop ... 360W / 48 = 7.5A. This is at least my limited understanding. If I have got this wrong, I do apologise, but then please help me understand this better – Shalan Jun 10 '23 at 12:30
  • @MicroservicesOnDDD: I don't have tech specs for the SMPS, but the specs for the Victron SmartSolar is here: https://www.victronenergy.com/upload/documents/Datasheet-SmartSolar-charge-controller-MPPT-75-10,-75-15,-100-15,-100-20_48V-EN.pdf – Shalan Jun 10 '23 at 12:32
  • @MicroservicesOnDDD: The only other specs I have to share are on the LiFePO$ pouch cells themselves:

    Nominal Voltage : 3.2v .. Nominal Capacity : 25Ah .. Discharge Cut off Voltage : 2.5v .. Charge Cut off Voltage : 3.65v .. Charge Current : 25A .. Discharge Current : 25A .. Peak Discharge Current : 50A < =10s .. Charge Method : CC / CV .. Cycle Life : 3000 (80% Capacity remaining)

     – Shalan Jun 10 '23 at 12:42
  • @user253751: yeah, the SMPS I have does not have a CC feature, hence needing to use an additional component – Shalan Jun 10 '23 at 12:46
  • @MicroservicesOnDDD: 100% agree with you on the GIGO ... but as mentioned, besides the specs I've shared above, and now the MPPT and LiFePO4, I don't have any other specs available unfortunately. – Shalan Jun 10 '23 at 12:48
  • If [W/V] wrong alright as a current limit, but step-up efficiency won't be 1: If the SMPS is current/power limited, you won't even reach 7 A. If you do, the PSU is slightly overloaded. – greybeard Jun 11 '23 at 01:40

1 Answers1

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This is a reasonable idea. It's how I charge my 12 V LiFePO4 battery from my car.

With the voltage set to 40 V and the current limit set to 5 A, the boost converter does approximate the output of a solar panel. You could even use (beefy) diodes to connect both the converter and the panels.

BUT the boost converter with constant voltage and constant current modes is a pretty good battery charger itself. Connect it directly to the battery, set it for the correct float voltage, 28.6 V or so, and set the current limit to as much as your battery or the boost converter can handle.

Read the battery datasheets to choose a safe constant voltage - it won't be as high as a smart charger will use, because that will drop back after some time.

Watch out, cheap converters like this are notorious for exaggerated specifications. Without a fan blowing on it, I'd say use it at no more than half of its maximum current.

tomnexus
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  • Thank you tomnexus! Regarding adding the resistor that you mentioned. Where in my diagram above would this go? Secondly, how do I determine what size resistor to use? – Shalan Jun 10 '23 at 12:53
  • I've changed my mind - the boost converter is a perfectly good battery charger. Just set it up and connect it to the battery. I'll edit my answer. – tomnexus Jun 10 '23 at 22:47
  • Resistor would go on the converter positive output, and choose something that doesn't waste too much power. Maybe 0.25 ohms, will drop 2 V, and limit peak current from a 10 V initial difference to <40 A, probably ok for a very short time. – tomnexus Jun 10 '23 at 22:57
  • Based on my experience playing with similar boost converters used for LiFePO4 charging, don't set it all the way up to your 29.2V mark. Give yourself a tenth of a volt or two as buffer in case of drift. You won't give up much in SOC and you'll keep for sure on the safe side of the line. – KMJ Jun 11 '23 at 01:39
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    Meant 28.6v, brain fart. Be a tenth below it, unless you have an accurate way to measure closer, IMO. Of course I'm not battery engineer, just a nerd building packs, so take it with a grain of salt. – KMJ Jun 11 '23 at 01:48
  • So @tomnexus & KMJ, you guys are both suggesting to connect the boost converter directly to the battery, instead of running this through the MPPT charge controller? – Shalan Jun 11 '23 at 09:36
  • @KMJ yes some buffer is probably wise. Especially as this battery has no BMS. The cheap boost converter will drift with temperature etc. This is one advantage of retaining the Victron - it's a much more trustworthy Li charger, properly engineered. You'll sleep better at night. – tomnexus Jun 11 '23 at 16:36
  • @shalan maybe start with the Victron manual. It might allow direct DC input to the solar port. Maybe even at 12 V, maybe minimum 32 V, maybe it will work correctly with a hard voltage source and just take Imax from it, no more. The whole idea of emulating the solar panel is really for simpler "PWM" controllers. – tomnexus Jun 11 '23 at 16:38
  • @tomnexus: the battery does have a 60A BMS connected to it. Sorry, forgot to mention that. – Shalan Jun 12 '23 at 08:01
  • @tomnexus I'm going to go ahead with your suggestion! – Shalan Jun 13 '23 at 13:22
  • @shalan do read all the comments and answers to this question before you go with the direct-charging solution. TLDR: It is fine for quickly topping the battery up, but for long-term standby on mains, it's probably better to let the Victron manage the charging properly. – tomnexus Jun 13 '23 at 16:51