Some truths, myths and opinions about “Drop In Replacement” LiFePO4 batteries…
I admit I have a bit of a bee in my virtual bonnet about this concept of a Drop In Replacement (referred to as “DIR” hereon). So let’s address the pro’s and con’s, because there are a few.
For those of you who like analogy’s, this is a bit like putting 91RON fuel into your vehicle that’s rated for 98RON. Sure it’ll work, it will get you to your destination (slower). But when you call upon the engine to deliver full power, it can’t, and when a warranty claim happens, they may well tell you you’re not covered because it was used out of its specification.
These DIR’s are normally a 12v (technically 12.8v) LiFePO4 cell pack, with a BMS there to protect it, sometimes including a balancer, and encased in some sort of standard sized plastic box with terminals to connect to. This makes them look like they will be a perfect DIR for your existing Lead Acid, AGM or other chemistry deep cycle battery. I will call them AGM from here as it’s the most common type for most people using a “leisure battery”.
While there are massive benefits to going to a LiFePO4, a DIR will be better, but if you use it as an actual DIR, you will miss out on many of those benefits. Charging a regular AGM battery has several “stages”. Most chargers will have 3, but some claim up to 7 stages. The charger should also factor in the temperature, because these batteries are very affected by it. Using a system that is designed for an AGM, will charge a LiFePO4, but not very well. It will take longer, and you’ll lose many of the benefits of using this battery type.
Charging a LiFePO4 has just 2 stages, Bulk and Float, nice and simple. The bulk puts power in fast and brings the battery up to full charge as quickly as possible. In some cases this bulk charge can be used as an equalization and absorption charge too, but that’s very dependent on the features of the BMS. The float stage is effectively a power supply while the charger is able to supply enough power for whatever the load is using. The settings for these vary according to the battery, BMS and Equaliser/Balancer (if installed). However in general, a Bulk charge voltage should be 14.2v, with between 14.0v and 14.6v being common. Float voltage of 13.5v is good, with 13.4-14.2v being common. However now isn’t the time to go into those nuances.
So what happens when we Drop In a DIR battery, into an AGM system. Well to start with, the charging current limits may be a lot lower than the LiFePO4 can accept. While this is normally not the case (as it’s usually limited by the solar capability), sometimes it can mean much slower charging than is possible. So where a half day for a LiFePO4 devoted charger may fully charge the batteries, an AGM version may not manage this. Some chargers have equalisation voltage settings, or can go higher than 14.6v. The next thing that can happen, is that it may go into Absorption mode which slows down charging for the last 20-30%. Again, this has the same effect, your batteries could be charged up faster, but it’s being limited by the charger. And after that, it may not be triggered back into Bulk mode at the right level to keep the battery full. Finally, if you have a battery monitor on the charger, it will very likely show a very incorrect charge status.
BUT… What’s worse is when people use a DIR as a second battery with a Voltage Sensitive Relay or other similar controller in a vehicle. A LiFePO4 presents a VERY low resistance to the alternator, especially when it’s very low. When this happens, the alternator will be working super hard to try to charge the battery. They have been designed to do this, but only for very short periods to bring an AGM back to a good voltage after starting. Most are not designed to output full power for potentially hours on end. Doing this may work, but it’s more likely that you will either burn out your alternator, or it will limit the power by turning off and on while you’re driving. Neither of these defence mechanisms are acceptable in my opinion, they are both indicating you’re stressing the system above what it is rated to do. I’ve seen this happen while testing this concept in my own campervan.
So what’s the solution?
It’s actually very simple, you can use a DIR, but you must also ensure your charging system is capable of being set up to work with a LiFePO4 charging profile. There are heaps of units available that will do that now, so you’re not limited for choice. However do your homework to decide what suits your needs best.
What makes a DIR a DIR? It’s just the BMS, but it’s nothing special and it can cause issues in itself. If you go outside its parameters, it can turn off. It may just turn off charging, or discharging, or both, you won’t normally know until it happens, and by then it’s too late. This is true for EVERY BRAND of DIR battery.
So the end result of this, is that the so called Drop In Replacement are not necessarily suitable to just drop into your particular system.
