This will be for an unattended hilltop repeater site where all the RF gear operates at 12VDC. Current consumption approx 2A (receive), 15A (transmit). Average 4 hours transmit per day, so about 100 Ah daily.
For standby power, we’ve always used a single 100Ah Trojan AGM paralleled across an IOTA DLS-55 power supply/charger. An outboard LVD protects the battery from over-discharge in an extended power outage. When grid power goes away, the battery carries the loads for about 20 hours before the LVD trips open to protect the battery. When grid power returns, DLS-55 tops up the battery automatically and everything returns to normal with no need to visit the site. This has served us well for many years. Most utility power outages have been less than 20 hours in duration.
With the prospect of grid power becoming less reliable, I'd like to increase battery capacity enough to provide 5-7 days run time. Something on the order of 600-800 Ah. PV panels are impractical at this site. This will simply be a 12VDC UPS, charged by utility power. No inverter. Just batteries powering 12VDC loads directly. Low discharge rate (2A typical, 15A max) for several consecutive days. In the event of a long duration grid outage, we could bring a portable genset to the site and let it run overnight once a week to top up the battery bank.
We could just get more AGMs and parallel them for the needed capacity, but the radio equipment is housed in an older wood frame building. We’ll need to fabricate a custom solution to spread out the 600+ lbs of AGMs to avoid overloading the floorboards. Pretty sure it’s do-able but will require extra work and expense. The building is equipped with electric heat & AC and the temperature stays between 50-80F year round except during power outages, when it could be anywhere from 0-120F.
LiFePO4 has many attractive advantages, but the two main sticking points seem to be:
For standby power, we’ve always used a single 100Ah Trojan AGM paralleled across an IOTA DLS-55 power supply/charger. An outboard LVD protects the battery from over-discharge in an extended power outage. When grid power goes away, the battery carries the loads for about 20 hours before the LVD trips open to protect the battery. When grid power returns, DLS-55 tops up the battery automatically and everything returns to normal with no need to visit the site. This has served us well for many years. Most utility power outages have been less than 20 hours in duration.
With the prospect of grid power becoming less reliable, I'd like to increase battery capacity enough to provide 5-7 days run time. Something on the order of 600-800 Ah. PV panels are impractical at this site. This will simply be a 12VDC UPS, charged by utility power. No inverter. Just batteries powering 12VDC loads directly. Low discharge rate (2A typical, 15A max) for several consecutive days. In the event of a long duration grid outage, we could bring a portable genset to the site and let it run overnight once a week to top up the battery bank.
We could just get more AGMs and parallel them for the needed capacity, but the radio equipment is housed in an older wood frame building. We’ll need to fabricate a custom solution to spread out the 600+ lbs of AGMs to avoid overloading the floorboards. Pretty sure it’s do-able but will require extra work and expense. The building is equipped with electric heat & AC and the temperature stays between 50-80F year round except during power outages, when it could be anywhere from 0-120F.
LiFePO4 has many attractive advantages, but the two main sticking points seem to be:
- I understand for optimum health, LFP cells need frequent cycling - is this correct? How much degradation in performance and life span should we expect from holding them at or near 100% SOC most of the time and they might only be cycled a few times annually. They will seldom be needed but when grid power is lost, they must work. A buggy-glitchy BMS that inexplicably disconnects the loads for no good reason will not be suitable at this unattended site.
- During an extended grid outage in winter, the indoor temperature could fall below 32F. Is it worthwhile considering self-heating LFP batteries for these rare instances? When grid power returns, the site’s wall-mounted electric heater will begin gradually raising the indoor temperature, but the battery charger will begin trying to recharge the pack immediately. It might take 1-2 hours until the pack warms up enough for the BMS to allow charge current into the cells. Am I understanding this correctly? I know that some LFP batteries are not equipped with low temperature charge cutoff, so we'd have to be sure to select batteries that do provide it. Been noticing quite a few low cost Chinese-made LFP battery packs (cells and BMS in a plastic case) coming on the market. Some are equipped with low temp charge cutoff, others not. Wondering if the low temp charge cutoff in these Chinese packs should be counted upon to reliably disallow charging below 32F - or should a separate means be provided to prevent low temp charging? There will not be anyone present to detect and respond to any malfunctions of the BMS. It needs to be safe and just work, unattended.
- Stay with AGM or switch to LFP?
- Anyone have any experience using LFPs in this application?
- Could we still use the same Iota DLS-55 to charge a 600-800Ah bank of LFPs if we unplug the existing (AGM-optimized) Iota IQ-4 module and substitute Iota’s new LFP-optimized IQ-4 module? I realize it might take 12-18 hours to fully recharge a pack of that size with a 50A charger but that would be okay for us.
- Anyone have any specific make or model of LFP batteries they would like to recommend? I'm okay with building a pack from raw cells/BMS, too - did that once before in a 48V residential PV system that gets cycled every day and it works great, but this application is entirely different.
